By the words "He is," is he to be apprehended, and by (admitting) the reality of both (the invisible Brahman and the visible world, as coming from Brahman). When he has been apprehended by the words "He is," then his reality reveals itself.

In plain words: A person first grasps the Self by accepting that it is real. That means accepting both the unseen Brahman, the ultimate reality, and the visible world that comes from it. Once a person accepts this, the Self's true nature becomes clear to them.