What is VBR?
It is short for “Variable Bitrate”. Some MP3 encoder support this. When enabled the compressor will use a bitrate that would fit the actual passage. If it is a very complex part it will use a quite high bitrate and e.g. on silence or easy parts it will use a lower bitrate. So usually the average bitrate will be not as high as a MP3 of same quality with constant bitrate.