Monkey Man is a man who was raised by monkeys who taught him to "jump good".
The Monkey Man is a human who was enslaved as a child by Aku's robot minions, but left that life when led away by a tribe of blue-skinned, white-haired apes. As per the customs of their tribe, he learned to "jump good", allowing him to leap great and unnatural distances.
He comes across Jack eating, and steals his food. After apologizing, he offers to show Jack his tribe. Following the invasion by a rival tribe of red gorillas, which Jack fights off, he strikes a deal with Jack to show him how to jump good in return for defense training. Jack shows the tribe various methods of practical defense, ranging from traps to personal combat training. In return, the tribe teaches Jack to jump good via the application of heavy weights and constant training.
When Jack leaves, he hears the rival tribe return, and witnesses the Monkey Man and his tribe successfully fend off the invaders.