the phrase tat tvam asi ( you are that ) refers to
Tat Tvam Asi Tat Tvam Asi (Devanagari: तत्त्वमसि), a Sanskrit phrase, translated variously as "Thou art that," (That thou art, That art thou, You are that, or That you are, or You're it) is one of the Mahāvākyas (Grand Pronouncements) in Vedantic Sanatana Dharma. It originally occurs in the Chandogya Upanishad 6.8.7,[1] 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.