To move the post up in the line of order. Or as Urban Dictionary says: BUMP to bring up somebody's post typically by posting the word "bump" on a message board. Can also stand for: Bring Up My Post bump!
On most forums, adding a post to the thread brings it back up to the top of the forum so the OP doesn't have to start a new thread to bring attention to his listing. You could type in just about anything you wanted. As long as you hit "POST REPLY" it will bump the thread to the top of the forum it's in.