Basicly, if you see a topic on the Board in which you are interested but there isn't much discussion or you are just curious and want to know more you just log in like you were going to answer the query and instead just type "bump" so you will get responses e-mailed to you automatically.
bump means you are pushing the topic back up to the top of the board or back to the first page, hoping to get more responses.
I personally hate the bump. It's boring. so when I "bump" something up, I usually use a smilie of some sort instead.