यः साक्षिलक्षणो बोधस्त्वंपदार्थः स उच्यते ।साक्षित्वमपि बोद्धृत्वमविकारितयात्मनः ॥ २६ ॥
In plain words
The awareness whose whole nature is to witness — that is what the word "you" means. And being the witness is nothing but being the knower, because the ātman never changes.
Close to the Sanskrit
That awareness whose defining mark is the witness — that is said to be the meaning of the word 'you'. Witness-hood too is simply knower-hood, by reason of the changelessness of the Self.