The ratio of the number of bits that are transferred between devices in a specified amount of time, typically one second.

Bit rate is the same as data rate, data transfer rate and bit time. The frequency at which bits are passing a given “point”. A definition is given here: Definition of Bit rate (click on link)

The higher the bit rate, generally* the higher the quality of music. There's more audio information per second being passed to the player. eg: A bit rate of 128 kbps will have less information to pass to the player than a bit rate of 320 kbps.
320 kbps is the maximum allowable for mp3's. (though there's a few rare players that can encode at higher, these bit rates are not recognised for most mp3 capable players.)

mp3 is a lossy format meaning it throws away information to help it compress in size. If you extract a song from an audio CD into a lossless format such as (1) AIFF or WAV then the bit rate will be 1411 kbps. So you can see how much less data is being passed to a player when an mp3 is being played. There's other lossless formats such as (2) apple lossless, flac, monkey/APE. Apple lossless files are saved as m4a & will have a bit rate higher than 320 kbps (somewhere between there & 1411 kbps.)

I hope that helps you to understand. But also keep in mind that the quality of the encoder is very important. Sometimes a song encoded at 192 kbps can sound better than one encoded at 256 kbps due to the better quality encoder used. Lame is considered the best encoder. And VBR is better than contant bit rate.

1 min of music = (1) wav file = approx. 10 MB, or (2) ape/apple lossless/flac = approx. 5 MB, or (3) mp3/m4a = anywhere between 1-4 MB depending on bit-rate, stereo, joined stereo or mono, sample rate, etc.