"buping" a post is done like this.
For example, someone posted something really great about something for about 1 year ago, then it has come like 500 new posts sience that, so that post is WAY WAY WAY down on the list. If you post a reply to that post you bump it up to the top of the list because it's a new reply to it.
So bumping a post is what you do when you reply to a post so it gets to the top of board.