That refers to audio bit rate. mp3's & mp4's are generally converted down arguably from 1411 kbps to somewhere between 4-320 kbps for mp3 or mp4 audio. So it represents quality of audio file in effect. Anything above 160 is usually good. 128 can be reasonable depending upon quality codec used to convert it. I generally aim at between 192-320 if there's a choice.

Unfortunately there are some very dumb people out there who think converting an mp3 at 128 kbps to an mp3 at 320 kbps will improve its quality. In fact it does the opposite. mp3 is a lossy format which means each time it's converted it loses quality. There are some files out there that have been reconverted. I've found some & found that the 128 of same song sounded better than one at 320. duh!

BTW it is possible to search via bit rate as a search option in audio search. You can also filter audio results via bit rate. ie: click on top left of filter & show other options.