In plain words
Tat tvam asi has no special meaning of its own. Tat tvam asi is just that and this joined together. That is, it is experiencing oneness with the body, the form we experience, in the invisible Ātma, and attaining Divinity. After that they went on setting out the jīva, God and Prakṛti, these three too, as another side of the triangle. They also went on setting out the essence of these three as rakti, bhukti and mukti. Rakti. What is it? This world itself is rakti for us. Bhukti: for this body bhukti is vital. The Ātma is mukti itself in form. So Tat tvam asi went on teaching that rakti, bhukti and mukti are vital for every human being. Gross, subtle and causal. Not only this, they went on holding within it the principle of Prakṛti, so great for the world. They also showed that Tat tvam asi flows as an inner stream through past, future and present. So how easily they recognised this Tat tvam asi within humanness and went on proclaiming it to the people! A river flowed. The river that flowed reached the ocean. The water of the ocean turned to vapour. The vapour took the form of clouds. The cloud fell as rain. The rain flowed again as streams. These streams took the form of a river. The river so formed reached the ocean again and merged in it. But where did this river come from? Where does the water of this river come from? That the river does not know at all. But once it has merged: Tat tvam asi. We set out from this very place. We and this are both one: this is what they came to recognise then. We planted a seed in the earth. It took the form of a seedling. The seedling grew into a tree. On this tree came branches, twigs, leaves, unripe fruits and ripe fruits. But where did these branches, these twigs, these leaves, these flowers, these unripe fruits, these ripe fruits come from? Though their forms and names, their tastes and effects are different, they went on showing the truth that these and those, joined, are one. The people. The essence in that seed is saying Tat tvam asi. These branches, these twigs, these flowers, these unripe fruits, these ripe fruits, all of them came from me. On this basis they went on showing that the whole world is Īśāvāsyam idaṃ jagat.