If we were all without knowledge, or ignorant, then our government could do whatever they please and we would not even question them. Once we gain knowledge, and only then, do we become difficult to govern.
As such we can interpret this quote as the one light shows in each of our pools, or minds, and in each of our minds there is one source of light. This light is often a metaphor for enlightenment. Notice how the light, or moon, is already there, how it is extant in everyone’s pool, or mind.
We can learn to listen to our bodies, which will help our minds, and vice versa. What we do to one thing affects another. This is why our lives all matter, as what we do in life affects others, no matter if we understand that or not.
We are all Buddhas already, we were already of our true nature, we just have to return to it. We must understand this, and we must understand that we will only become enlightened when we stop trying to become enlightened.
Do not expect results, and you will get results. Smile not to make others happy, and it will make others happy.