A bit rate is a measure of bits transferred per unit of time.
As it pertains to music, bit rates are measures of the samples in an electronic music rendition. The de facto unit is kilobit per second or kbit/s. Common bit rates with mp3 files include 128 kbit/s (standard), 192 kbit/s (medium quality), 256 kbit/s (high quality) and 320 kbit/s (highest quality). A high bit rate does not necessarily make the sound better but a low bit rate (e.g. 32 kbit/s) sounds bad.