This question has been asked often in the SO Forums, and it doesn't have a thread of its own here (although it's hidden in at least one other thread).

Many threads have more than one post with the word "bump" in it. This is simply an expression used to refer to a post made simply for the purpose of moving the thread back up to the top of the page. In fact, any post you make on these boards bumps the thread to which it belongs to the top of the first screen of that particular forum. That's the way the software works.

It's primarily used for moderator or other informational messages so that they don't get "lost," and peeps see and read them. Occasionally, it is used for a thread that has dropped a few pages back when peeps are posting new, duplicate, messages that better belong on the original discussion thread.

It is considered "bad form" to bump your own message in the hope of getting (any/more) replies.

Adding new information to an older thread that has not yet reached the archives is also, by definition, a bump.

Thanks to MysticWolf for contributing greatly to this explanation.