"Bumping" a thread is when you post "bump," or otherwise add nothing significant to a thread, merely for the purpose of keeping the thread alive. It is prohibited to do so on DP and many, though not all forums.
There is also the cross-bump where people start a new thread linking back to the old one. It all boils down to this: A bump is the act of attracting undue attention. (Undue hence not allowed.) It's spam basically.