CC, Brahman is so great a mystery that even vedic philosophy has not been able to answer what exactly is Brahman. Here are a few statements from the vedic philosophy about Brahman.

Brahman can be unknown to the one who knows it and can be known to the one who doesn't know it.

Brahman is not that which the eye can see, but it is that by which the eye sees. Brahman is not that which people worship as an object.

Brahman is the eye of the eye, the ear of the ear etc.
The mind cannot reach It. The speech cannot express It.

Neither by reading the scriptures, nor by a 1000 good acts, nor by elaborate rituals, can one know Brahman. To know Brahman one must have absolute faith and must be free from all desires.

He whose desires have been burnt in the fire of Knowledge, is eligible to know Brahman.

What lies in the front is Brahman. What lies at the back is also Brahman. Brahman is also above, below, to the left and to the right.

The Brahman is the real Self which shines in its true glory after the destruction of ego.