The bit rate or bitrate of media measures the bits used to store the data per second (kilobits per second, kbit/s or kbps). It is formally defined as the product of (bits per sample) × (samples per second).
A constant bit rate or constant bitrate is a bit rate that does not change.
A variable bit rate or variable bitrate is a bit rate that varies depending upon the complexity of the data.
Some common bit rates for mp3 files are 128 kbit/s (standard), 192 kbit/s (medium quality), 256 kbit/s (high quality) and 320 kbit/s (highest quality).
A low bit rate (below about 128 kbps) makes for poor audio quality.