‘Tatvamasi,’ meaning ‘You are that is,’ was called Updesha Vakya, as it instructed a seeker on the nature of Brahman, the ultimate truth. That refers to existence of God in every one of us. However, factually, we are not God, but only a part and parcel of God
It originally occurs in the Chandogya Upanishad, in the dialogue between Uddalaka and his son Śvetaketu; it appears at the end of a section, and is repeated at the end of the subsequent sections as a refrain. The meaning of this saying is that the Self – in its original, pure, primordial state – is wholly or partially identifiable or identical with the Ultimate Reality that is the ground and origin of all phenomena.
When you meditate on God, you become one with God. . That is why, it is said, `Tatvamasi’, meaning,
“Thou art that”.