A "bump" is given to make the thread appear again on the main page, so more people will see it.
SugarFrosted summed it up perfectly. But if you want more info, it's pretty simple: when you post to a thread, the thread moves back up to the top of the list of topics in its forum. Even if it doesn't manage to hit the home page, bumping it it to the top of the forum will make it more likely to be read.
People write "bump" when they don't really have anything to add to the thread but want to see the post back at the top, either because the post didn't get answered so they want to keep it from getting lost on p.2 or 3, or some people "bump" it so they can find it easily in the thread.
Not that you asked (but in case someone else is reading this), bumping to keep it easily found isn't necessary, instead you can just hit the "Watch this topic for replies" link at the bottom of the thread and it saves the thread into your My Watched Topics list. Some people don't realize they can do this and instead you'll see "Bump's" cousin: "Save" 
Don't save, just hit "watch this topic"!! 
LOL I think you should have stuck with SugarFrosted's answer! I have a tendency to run off at the keyboard 
. . . Bring . . . Up . . . My . . . Post . . . BUMP . . . 
. . . (we are encouraged NOT to bump . . . except the original poster if there is no response to your post) . . . HTH
Just to be clear, the homepage is not the best way to find new posts. The homepage view of recent posts is merely for new visitors to see what is being talked about at CC.
The best way to view new posts is to use the VIEW POSTS SINCE LAST VISIT link at the top of the forums.
The ONLY TIME you should use BUMP, is if several hours have gone by and you have not received any response.
We will actually go in and delete unnecessary BUMPS
It's can also be abused as a cheap way to raise your post count. I really dont' recommend it as previous posters have stated. It's a very bad habit to get into. imho
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