VBR stands for 'Variable Bit Rate', as opposed to the standard option of 'CBR' or 'Constand Bit Rate'.
What is the difference you ask?? ?
CBR has been fairly widely used and thus most mp3 enabled devices and software can play it, whereas some devices may not be VBR enabled (older devices I expect).
CBR uses the same encoding bitrate regardless of the music quality, so, sections where there is little sound or dynamic range still encode at a higher than necessary bit rate. By the same token, sections with alot of data and dynamic range may not be permitted additional bitrate necessary to avoid losses. This allows for a predictible file size, but quality may vary throughout the song.
VBR focuses on quality, adjusting the bitrate as necessary to encode the dynamic range, peaks, etc at the exoense of file size being unpredictable. As the source material increases in range the bitrate increases, in the quieter passages the bitrate is reduced accordingly to maintian the quality without unecessary space wasted.
VBR is a little slower to encode than CBR.
VBR can produce better quality, but filesize may be a bit larger.
Best advice, encode the same music with both and see what your ear says.
Personally I use VBR as I value quality more than filesize.