A chamber in a bong in the bottom part of the bong, usually, where the smoke fills up, when it fills up, you release either the carb hole, or lift the stem tube up. Go look at the Bong Thread in the pictures section and you'll see what I'm talking about.

The design can be anything you want. It can be a big chambers or a little chamber. They're like mini smoke chambers, where the smoke fills up.

Anyways, hope this made some sense.