It means variable bitrate. Usually better than 192 kbps but not as good as 256 or 320. Sometimes referred to as VBR and CBR. There are at least two kinds of VBR encoding though, where one is quite good and the other not so good. I don't encode to variable though so I can't tell you more.

320 constant bitrate is the best mp3 format there is. 256 a close second.