Variable bit rate (VBR) encoding is an alternative to constant bit rate encoding and is supported by some codecs. Where CBR encoding strives to maintain the bit rate of the encoded media, VBR strives to achieve the best possible quality of the encoded media.
The quality of encoded content is determined by the amount of data that is lost when the content is compressed and decompressed. Many factors affect the loss of data in the compression process, but in general, the more complex the original data and the higher the compression ratio, the more detail is lost in the compression process.

To convert a movie encoded with VBR to CBR you could use the following programs:

VirtualDubMod
BeSweet
MusicMatch Jukebox 7