What is Brahman? This is a difficult question which even scriptures have to grope to answer, because Brahman is so subtle and stands well beyond all what is reachable to human mind, intellect and intelligence. None can understand the infinite significance of Brahman.
The Upanishads merely state that Brahman has no legs but can walk, has no hands but does all the work, has no eyes but sees all, etc. Realisation is a state when such a theoretical explanation is grasped in its subtle and infinite nature, when in the inner being one is able to experience the infinite nature of Brahman.
Scriptures reiterate that only a true preceptor who is himself realised can enable a sincere seeker to realise Brahman, pointed out Sri O.N. Ravi in a discourse. The lakshanas of a guru and his prabhava are detailed by Lord Siva to Parvati in the Skanda Purana. The qualifications of the ideal disciple are also explained. The syllable ‘gu’ stands for darkness or ignorance; and ‘ru’ stands for redemption from ignorance. Therefore, a guru redeems his disciples from the ignorance of Reality.
Sri Ramakrishna tries to drive home this point through a simple illustration. A father has two sons and both are given training in the Vedas. At the end of the training, when the father asks the elder one to disclose what he had learned about Brahman, the boy is able to quote the Vedas and the Upanishads, the Srutis and Smritis, etc. unerringly and in a perfect manner. When the father asks the other son, the boy merely remains silent, even after repeated appeals to explain what he had learnt. The wise father interprets this silence as proof of the son’s grasp of the truth of Brahman since it is a fact that none can express this truth adequately.