Dick is a vulgar slang word for a penis. It's also an insulting thing to call someone you don't like, especially a boy or man.
Much like ass, this word for a below-the-belt body part is very commonly used as an insult meaning pretty much "jerk" or "idiot." Only males have dicks, and this insult is usually used toward a guy who is "acting like a dick." Lots of people find this word offensive, especially parents, teachers, and other adults. If you have to discuss a dick, you're better off saying penis or using a slang word that's sillier and less offensive, like johnson.