Posts are listed in order of when the last person posted on them. If no one posts on a thread then it drops down the page and eventually off of the first page, often never to be heard from again. On this board is isn't as big of a deal but on some of the fast moving boards a thread can be lost to the 2nd or 3rd page in a matter just minutes if no one posts on it. So often the original poster or someone else will post "BUMP" just to push it back to the top of the first page so it will be noticed by people scanning the boards an hopefully get a question answered or a story read.
Does that make sense?