Bit rate is the encoding rate of the songs. mp3's can be encoded up to 320 kbps. Likewise for m4a files, however if you see an m4a file at higher than 320 then chances are it's a compressed lossless file (ie: no loss of quality & same as an aiff or wav except it's been compressed without throwing away information like mp3 does.)

320 is best, 192 also good. Anything under 128 is considered as cheap & nasty so to speak. Too much info tossed out in order to save space during the encoding process.