the number of bits of information recorded for each sample
Mono: Mono data uses one channel.
Stereo: Stereo data uses two channels
In this mode, the encoder will try to maintain an average bitrate while using higher bitrate for the parts of your audio that need more bits. The result will be of better quality than CBR encoding while the average file size will remain predictable.
With variable bit rate AACs, the encoder automatically detects which bit rate is most appropriate for the sound being encoded.