Variable bit rate, as opposed to constant..

Does the motion in the video determine how big the file size is going to be?

Yes.

VBR and CBR actually work in pretty much the same way, except VBR's compression of non-changing sections of a frame is way more aggressive.

Oh yep, so the compression is more relaxed when there is more activity going on yeah?

Kris do you know why you can't do 2 pass encoding with CBR?

I would imagine that during the first pass of two pass encoding it's collecting statistics on where the action is so it can choose where to spend its bits, given you probably specified a final file size.

Pretty much. Multipass means a more highly optimised file. CBR you have no need for that because you have a set bitrate. VBR tries to minimise the bitrate as much as possible per your settings.