I agree with Narcissus about Brahman cannot be explained.
I don't agree with Narcissus about the need for a Guru.
Brahman can be understood only by experiencing Him/It, while no Guru can give us that experience. This is what Sri Sankaracarya says (and what others say).

In words, Brahman is the godhead -- the un-manifest, absolute, transcendent, all-pervading source of all manifest creation. Brahman is described as Sat-Chit-Ananda, which means pure being (or pure existence), pure consciousness and peace (wrongly translated as bliss).

BTW, Brahman and Brahma are two different things.
Brahman is the godhead, while Brahma is a God.