As each thread recieves a new post in it, like this one, it gets 'bumped' to the top of the list. If no one adds a new post, then the thread slowly works its way to the bottom of the list, then it disapears. It can still be found in the 'forum' section, but if someone has something for sale, or for some other reason they want the thread to remain on the front page for a bit longer, then they add a post of their own. They are just 'bumping' it up to the top of the heap again.

The word 'bump' [or sometimes 'cough'] simply acknowledges that the sole purpose of the new post was to push that thread back up there for all to see for a bit longer.