More specifically... bitrate is a measure of how many bits are used to store musical information over a given period of time. For example a bit rate of 128kbps means that one second of audio will consume 128,000 bits of storage. Higher bit rates have better sound quality.
So... the admins have decided to use the sound quality standards to label the users based on how many posts they have. For instance, Confused is labeled FLAC. That's a compression that is virtually lossless, meaning the decompressed sound has as much information as the compressed sound.