। ॥ २५ ॥
In plain words
What the sentences of Vedānta make known; what stands beyond the whole universe; imperishable, without a second, utterly pure, and lit by nothing but itself — that is what the word 'tat' is pointing at.
Close to the Sanskrit
That which is to be known through the sentences of Vedānta, which is beyond the universe, imperishable, non-dual, pure, and known by itself — that is the implied meaning of the word 'tat'.