The higher the quality, the lower the amount of compression and the larger your exported file will be. If you choose a frame rate that's lower than the movie's current frame rate, compression will cause frames to be deleted. If you choose a number that's higher than the movie's current frame rate, existing frames will be duplicated. In most cases, QuickTime uses frame differences to compress moving images. The key frame contains all of the information for an image.
Subsequent frames contain the information that changed.
You can specify how often you want key frames to occur. If you don't have enough key frames, the quality of your movie will be very low because most frames are generated from others. On the other hand, more key frames means a movie that is larger and has a higher data rate. How often you set key frames to occur depends on how your movie will be played, and how much of the movie changes from frame to frame. If the movie you're exporting will be played from a CD-ROM disc or over the Internet, you can choose fewer key frames.