The brain is the most complex organ in the body, located inside the protective skull. The word can also be used as a verb in the phrase "to brain," which means to injure or kill someone by hitting them in the head.
The human brain has inspired many idioms, or colorful phrases. Thinking intently (or obsessively) about something means that you've got it "on the brain," and to "pick someone's brain" means to get their ideas on something. A puzzle is a "brainteaser, someone smart is called "a brain," and the person who runs something is said to be "the brains" of the operation.