Well, I don't know a lot about this, so I'm sure someone could give a much better explanation. But, in an mpeg video, not all video frames are equal. The I frames are like key frames, containing lots of video info. The P and B frames are compressed frames and only contain video info that is different from frames that came before or after. The more I frames, then the better the quality. But it's a tradeoff between quality and compression. In high action scenes you'd need more I frames, but since they contain much more info your bitrate goes way up. So overall you've got to strike a balance. And that's about the sum total of what I know about frames
