When someone "bumps" a thread, they move it back to the top of the thread list (thereby increasing potential posts in their thread) by adding an irrelevant post, or "bump" to their thread.
Also, it means that if you're interested in the topic and want to hear other DISers answers if you "bump" you get replies sent to you as well and you can hop back to the thread and read their answers.
When you post to a thread, it goes to the top of the list. If it is at the top of the list, DISers are more likely to see it and respond to it than if it is down on page five.
Often people will use 'bump' if they aska question and haven't gotten an answer, or haven't gotten a complete answer. They don't want to post an asnwer tot their own question, so they type 'bump.'
Sometimes folks bump really old threads just to see them revived.