A mullet is a kind of fish. Mullet often jump out of water. People are not sure why, but it might be because they are trying to see where they are going.
In recent years, "mullet" also came to be a slang word for a hairstyle that is short at the front and top, and long at the back. This slang word is said to have been started by Mike D of the Beastie Boys in 1995, and after 2000 was spread by a small group of people in San Francisco through the Internet.