In short, this expression refers to the profound vision of reality where one perceives all beings, from the smallest insects to the gods and everyone and anything else, as your mothers. In Buddhist cosmology, regardless of the tradition, culture or time period, time is seen infinite because otherwise it would violate the observable law of conventional causality – that is to say things arise due to causes, and those causes in themselves must have their own respective causes as well. This rules out a creator deity not subject to causes yet still responsible for creation of time and space. On the same point we can indeed infer that there is no start point in time because that start point, if it did exist, would likewise occur without a cause, which is fallacious. Thus we say the past is infinite.
Now, given that the past is infinite, and that as a sentient being you have been reborn infinite times, it logically follows that all other beings have at some point been your mother, who unconditionally showed compassion and loving kindness toward you.
This is an extremely profound vision of reality that all too often gets lightly tossed around. There are many who utter such a phrase, but it hardly reflects in their behaviour towards others. If you truly felt even your fellow humans were your mothers, would you ever talk down to them, intentionally work against them or show them contempt? I'm guilty of this myself, but the more I cultivate this sense of unconditional gratitude the less contempt and anger I feel towards others – even those who have intentionally harmed or cheated me.
This vision forms the basis for unconditional compassion and gratitude towards all beings. This assumes of course you have conviction in rebirth. If you don't actually think of rebirth as realistic, then seeing all beings as your mothers will prove fruitless. It will just be a novel visualization that amounts to a religious fantasy rather than being a transformative experience. Being that most individuals brought up in industrialized cultures have a materialistic view of their own existence (meaning they do not think of rebirth as realistic, even if they proclaim a “belief” in it), I fear this kind of practice will be largely ineffective. This is unfortunately just a sign of our degenerate age. In Hindu thought, which curiously in some circles predicted the eventual rise of widespread materialism, this is the kaliyuga. In Buddhist thought the term kaliyuga is utilized in Tibetan Buddhism while elsewhere alternative terminology is employed such as the “dharma ending” age (Chn. mofa / Jpn. mappō 末法). Basically, that religious practice is becoming increasingly ineffective due to increasingly defiled minds and the proliferation of wrong views is a sign of our degenerate age.
That being said, it isn't impossible. This sort of approach to sentient beings, from lowly insects at your feet to both your good friends and hated human foes, also leads one towards an even greater vision where all beings are not just your mothers, but are seen as buddhas (i.e. fully enlightened, ultimately compassionate and completely free of any and all mental afflictions). Allow me to quote the famous Indian Buddhist thinker Shantideva.
“The great compassionate lords consider as themselves
All beings – there's no doubt of this.
Those whom I perceive as beings are Buddhas in themselves;
How can I not treat them with respect?”
Hence it is also said that a bodhisattva prostrates themselves before all beings. This is not for some superficial demonstration of having rid oneself of pride. This is both an expression of utmost gratitude towards all beings who in the infinite past have shown you great kindness, but also you recognize that a buddha, having taken the universe as its body, essentially coalesces with all resident beings in reality. As Shantideva states they “consider as themselves all beings”. In China there is a story about a monk who bowed before everyone he met, considering them buddhas. Whether or not this truly happened or not is unimportant – what is important is the ideal. Genuine humility and reverence towards everyone. Again, it must be stressed that such a perception of reality is extremely profound and I think for most individuals, myself included, this takes many years of practice, if not several lifetimes. The result would be eradicating from one's mind all contempt, hatred, jealousy and ill-will, while simultaneously really thinking of other beings as just as important as yourself, if not more.
So, how does one cultivate such a vision? I am of course not enlightened and cannot provide definite instructions to the end of path I myself have not taken as of yet. However, a journey of a thousand miles begins with a single step under your feet, and I would hopefully be able to say I have taken that step.
I think that first step is simply stopping to consider – seriously consider and think over – the kindness others have shown you. There is no seeking of a metaphysical undercurrent or contemplation of philosophy necessary for this. One need only reflect on the fact that countless individuals, and perhaps even some animals you might be acquainted with, have indeed been kind and generous towards you.
They might not always have done so – on the contrary, they might have harmed you at some point. However, in such a situation keep in mind that when struck with a stick one does not blame the stick, but the purported agent wielding said weapon; meanwhile one is unaware that the agent is only carrying out this action because of afflictive emotions active in their mind. You don't blame or resent the stick because you think the agent wielding it is responsible. However, why blame the agent when they're only behaving as such due to afflictive emotions. Blame those defilements, not the person. In the same sense you do not blame a senile person hurling slander your way because you recognize their actions are dictated by a painful and degenerate state of mind. Anger, which drives the agent to strike you with the stick, is basically the same thing: a degenerate state of mind.
In any case, reacting with similar sentiments of anger or worse violence will only harm oneself. As Milarepa said, “If you abhor your own suffering, cease harming others.” This also means cease returning blow for blow. It only leads to your own misery.
Having considered the generosity and kindness others have shown you, consider the potential that in past lives at some point all beings have indeed shown such good-will towards you. Given the fact of infinite past as outlined above, we can indeed infer this to be the case. This should summon a sense of gratitude. We never hope harm will come to those who have been good, honest and generous with us. We wish them well and want to repay their kindness with positive deeds. Having expanded this sense of gratitude to encompass all the people we encounter, both familiar and stranger, we might let it include our animal and insect friends as well. I know that might sound odd to many readers – “insect friends” is not something you usually say (at least not without the risk of being called a lunatic), but it is possible to cultivate a sense of good-will towards those creatures in the world so unlike us in terms of genetics, physiology and size.
One then puts this into action. Feed stray dogs. Restrain yourself from killing annoying flies. Give generously to those who are in need expecting no recognition for the good deed as it really is just reciprocation for their past benevolence towards you. Be like Milarepa who let the maggots have their share of his food despite being destitute and on the brink of starvation in a cave.
If you can seriously accept the good deeds all beings, without exception, have shown to you in this and past lives, then recognizing them as your mothers will prove an easy further step to take, which further can lead to seeing even those cockroaches crawling at the bottom of the rubbish bin as buddhas. At that point you can genuinely prostrate before all sentient beings and truly have no sense of pride or contempt towards anything at all.
Being free of all ill-will the conditions for negative karma to be enacted against beings will not exist. The result of negative karma is our own suffering and having eradicated the cause, the resultant effect cannot arise.
Incidentally, this kind of practice has a benefit to meditation as well. It helps in eradicating the hindrance of ill-will, which prevents cultivation of meditative absorption. Hindrances like desire, doubt and so on also need to be addressed, but ill-will is something which plagues our minds, though it is of course possible to overcome.
Thus, to think “all beings my mothers” is not mere sentimentality, but a spiritual practice that can be employed to rid oneself of ill-will, which is a factor in creating negative karma, whose result is always one's own suffering, as well as doing away with a hindrance which prevents deep meditative stabilization, which is necessary for cultivating the mental fitness necessary for realizing emptiness.