VBR is a way to make an mp3 file smaller, but still maintain a high quality of sound. In contrast to CBR, the kbps changes during the song that has been encoded using a VBR. The VBR will use a higher kbps on particular complex parts of a song, while dropping the bit rate during simple parts of a song (like silence). For example, a 320 kbps constant bit rate song with be 320 kbps even during relatively silent parts of a song. Some people think that is wasteful, so a VBR will drop the kbps during these parts. In effect, you get a smaller file size, or as some put it, a higher ratio of quality to size. The actual sound quality between a VBR and a CBR is supposed to be humanly unperceivable.