Variable Bit Rate is an encoding method that is designed to achieve a better sound quality vs. file size ratio than CBR (Constant Bit Rate) encoding. This is achieved by continuously changing the bit rate during the encoding process depending on the nature of the audio. For example, if there is silence to be encoded then the bit rate is lowered to optimize the file size. In contrast, if the audio to be played contains a complex mix of frequencies then the bit rate is increased to give good sound quality.
Using the VBR encoding method will produce an audio file that will have variable bit rates from 128Kbps to 320Kbps depending on the complexity of the audio frequencies.