Threads come back to the top of the list of threads whenever someone replies to it. When someone wants a thread to stay close to the top of the list but it has become burried on the bottom of page one or moved to page two or further, they will go in and just put "bump" in as a reply, thus moving the thread back to the top of the list.
The word itself doesn't move it back to the top but replying does and folks see the word "bump" and know what's going on.
Sometimes, instead of "bump", folks will just reply with one of the smilies of some sort. Same effect, though.