Your ping accounts to information you send/recieve over the internet. Usually, your ping is ok if Halo is the only internet app you are using.
Now, there are a number of things that can cause your ping to shoot up while playing Halo. First, it could be totally out of your reach, as your high ping can be caused by another player with an abnormally high ping, and is a result of communicating with them. The main cause, however, is having other internet applications open while playing Halo. For example, having a file sharing program open while playing halo will cause your ping to shoot through the roof. Off of the BFM servers, I like to josh players that have pings around 1000 by saying "Hey ________, quit downloading (uh, you know) while playing Halo!"
Another cause could be your internet itself. If you have a slow dial-up or out of date modem that can't keep up with the fast broadband motems of today, your ping will also be really high. And the only way to fix that is by upgrading your internet.
Those are the two most common causes of high ping, so try them.