What is Brahman?
In the Mundaka Upanishad, Chapter II, Section 2, it has been stated:
"The Omniscient, the all-wise, whose glory is reflected here on earth, is the self-enthroned in the luminous city of Brahman, his etherial heaven. Firmly established in mind, seated in the heart he controls life and body. The wise by the higher knowledge see him clearly as the radiant, blissful, immortal."
The explanation of the above in the Upanishad is that the Brahman is "self-resplendent and luminous" and we see the manifestation of its lustre on earth in the blue skies, the sunrise, the stars, the green of the forest.
In the human body, he is seated in the brain where he builds up thought and in the depths of the heart from where he controls life and body.
"The force that resides in the tiniest atom with the capacity to destroy a city is the same whose smile kindles the Universe."
In normal consciousness, the Brahman can only be seen as stated above but not truly realized. He can be thought about or written about at this level of understanding. However, for the realization of the Brahman teaches the Vedanta, what is required is higher knowledge or "Brahma-Vidya". This knowledge can be obtained through solitude and meditation.
When that knowledge arises and awakens us, then the "Great Being shines forth through every pore of our being as the blissful, the Immortal."