VBR or Variable Bit Rate is a term used in computing and telecommunications that relates to the bitrate used in video or sound encoding. VBR files differ the amount of output data per time segment contrasted with constant bitrate (CBR). While a smaller amount space is allocated to less complex segments, VBR allows a higher bitrate (Hence needs more storage space) to be allocated to the more complex segments of media files. The average of these rates can be calculated to produce an average bitrate for the file.   

VBR encoding regulates the data rate down and to the upper limit you set, depending on the data required by the compressor. This means that during a VBR encoding process the bitrate of the media file will dynamically decrease or increase based on the media files bit rate requirements. VBR produces the most favorable results even though it takes longer to encode as the quality of the media file is superior. Most commonly, VBR is used for http delivery of video content.