VBR, or Variable Bitrate, describes a method of encoding that allows a different number of bytes to be used for different portions of a video when compressing the file size of the video.
The visual complexity of a video can vary dramatically from frame to frame. Consider a scene of race cars zipping around a track with lots of smoke and movement, that then switches to a closeup of a fan watching the race, with almost no movement and a clear blue sky in the background. To achieve the same visual quality when compressing the two shots, the shot with the race cars would take many more bits than the shot with the fan. VBR encoding allows the encoder to adjust the number bits it uses as it finds different levels of complexity in the video. This is in comparison to CBR or Constant Bitrate encoding, where the bitrate remains consistent throughout the video despite any changes in video.