What is "BUMP"

In a forum like this with so many people if you have a question that needs answered or something for sale. It can and will move down the charts quickly and the next thing you know it is on page 4 and then gone into oblivion.
When you respond to a post/thread it moves/ BUMPS the thread to the number one spot on page one. Some have nothing to add to the thread and just use BUMP. This forum does have rules regarding personally "BUMP" bumping though.