Bit rate

Bit rate is the amount of data to be read when playing a digital audio file.

The unit is kbs=Kilo Bits per Second.

Don't mix it up with bit depth; the number of bits per sample.

 

Uncompressed PCM audio e.g. WAV in CD quality is playing 2 channels with 16 bit word length and 44100 samples per second = 2*16*44100=1411200 bit/s or 1411 kbs.

In case of linear PCM, the bit rate is simple the product of sample rate * bit depth * number of channels.

 

FLAC is lossless compression. The amount depends on the complexity of the signal. If you compress CD quality using FLAC a values of approximately 900 is not uncommon. As it is variable bit rate by design, higher and lower values are possible. Of course, if you play it, it is expanded again to linear PCM so a 2 channel, 16 bit with 44.1 kHz sample rate. The bit rate of a FLAC tells you how efficient the lossless compression has been compared with the original source.

In case of lossy compression e.g. MP3, the bitrate simply becomes the target. A 320 kbs MP3 is obtained by removing as much information until this target is obtained. On playback the decoder expands it again to 2 channel 16 bit 44.1 kHz but the information lost in the lossy compression, is lost forever.

Why do they use the bit rate of the file instead of the actual playback rate?

The answer is simple, the actual rate is not very informative.
You rip a CD to WAV, FLAC and 320 CBR MP3.
To play them they must be converted to something a DAC does understand, linear PCM.
This invariably will reproduce the values of the source, regardless the format used.