The thread goes back to the top of the list where people will pay attention to it if someone posts in it. Otherwise it keeps getting pushed further down.
People "bump" a thread if no one has responded and they want to bring it back to the surface for another try. They might be asking an unanswered question and so they have nothing to add, so they just say bump.
It means posting just to move the topic back up to the top of the list, so it will get more exposure.
It's definately frowned upon, because it means someone else's topic gets shoved to the bottom and left behind.
In certain cases a bump should be allowed. Sometimes a forum moves so quickly it doesn't take long for a thread to push down. You have 3 choices. Let it go, without gettting an answer. Bump ~ to try and get help (pushing other threads down) or start another thread ... which also shoves other threads down and then gets deleted for being a duplicate thread without ever getting a chance to get an answer. *shrug*
In busy sections of BYC a thread can be pushed to the second page within an hour and a half.
In the emergency section i fyou post an emergency and don't get a response say within an hour, you can bump to the top. However, it must not be excessive and and a reasonable period of time must go by.