This, which rests eternally within the self, should be known; and beyond this not anything has to be known. By knowing the enjoyer, the enjoyed, and the ruler, everything has been declared to be threefold, and this is Brahman.

In plain words: The reality that always rests inside you is the one thing worth knowing, and nothing beyond it needs to be known. Everything is made of three parts: the one who experiences, what is experienced, and the ruler over both. All three together are Brahman.