Brahman means absolute reality or cosmic spirit and is the Hindu concept of god. It is derived from the root brhi+suffix manin, where brihi means to grow, to expand, in other words it means the all encompassing, infinite and absolute. Hence why Brahman is said to be uncaused, beyond comprehension, description, space, time and causality.
In the Brahma sutras Brahman is described to exist in two forms(similar to Prakriti) Nirguna Brahman, meaning Brahman without qualities, descriptions or attributes and Saguna Brahman, meaning Brahman with qualities, descriptions and attributes. In order to illustrate the difference consider the image of god as father; because this gives god the anthromorphic quality of fatherhood, it is considered Saguna Brahman. Saguna Brahman can also be given natural and animal qualities, hence why Hindus also have images of god as animals and natural elements like earth, air, fire, water, ether. This is why Hinduism is said to have innumerable gods, but in actual fact this is errorneous, rather it is innumerable Saguna Brahman concepts.
Nirguna Brahman is beyond description so it is left as a mysterious or absolute reality. The less said about Nirguna Brahman the better for it is beyond the comprehension of the human mind. It is beyond duality. It is beyond language. It cannot become objectified for it is the reality underling even the observer. However, what is said about Brahman by the mystics is that it is satchitananda meaning pure existence, pure consciousness and pure bliss. In some ways it is very similar to the mahanirvana concept in Buddhism.
The concept of Brahman is first described in the Vedas, elaborated particularly upon in the Upanishads which formulate the philosophy known as Brahmanism. Later, the formal Hindu school of philosophy Vedanta arises to give a rational demonstration of Brahmanism.