what is brain
Again, you need to be more specific.
Do you want to know what it does or etc?
If you want to know what it does, I'll post that below.
From wikipedia:
In animals, the brain is the control center of the central nervous system, responsible for behavior. In mammals, the brain is located in the head, protected by the skull and close to the primary sensory apparatus of vision, hearing, equilibrioception (balance), sense of taste, and olfaction (smell).
So in a summary, the brain controls your bodily functions, such as your movement and it's what allows you to think and have senses.