In plain words
This is Satya. This is the law. This is the Ātma. This is Tat tvam asi.
A river flows and joins the sea. In the rays of the sun the water of the sea turns to vapour and takes shape as clouds. Drops of rain fall from the clouds. These rains take shape as brooks and streams. The streams, coming together, become the flow of the river. But the flowing river does not know that this water is its own. The water in the river, the water in the clouds, the water in the rain, the water in the flow: all of it is one! This is Satya. This is the law. This is the Ātma. This is Tat tvam asi.
Sādhana is the effort to recognise the all-pervading divine principle and to live by it
Here are some illustrations we meet in daily life. We take a little salt and put it in water. In a while the salt dissolves and spreads through all the water. Though the salt has spread everywhere, it is not seen by the eye. It cannot be caught by the hand. But one cannot say that it is not there. Only when we put it on the tongue does its taste become known to us. Though it has spread everywhere, we must not think it is not there merely because it is not seen by the eye and not caught by the hand. This taste that has spread everywhere is you. That is the Ātma. That is Satya. That is Tat tvam asi.
In the waking state man goes through experiences of many kinds. He forms ties and relationships of many kinds. He meets joy and sorrow, honour and esteem. But in the dream state all these come to nothing. In this dream he creates himself. He creates his joys and sorrows too. He creates his fears and delusions himself, goes through them himself, and falls prey to these fears and delusions. He creates his foes and his friends too. In the state of deep sleep he merges in the Ātma and experiences the infinite principle. Yet the one who went through the waking state is he himself, the one who went through the dream state is he himself, and the one who knew Ānanda in deep sleep is he himself!