Bitrate, often also written as bit rate, is a term often found when dealing with video and audio files. The Audio Bitrate in particular gives information about audio data. In audio files like MP3, this is obvious. In video files like MP4, it should be distinguished from the video bitrate since it only refers to the audio data of the file, not the visual data.
The Bitrate expresses the amount of bits processed over a certain amount of playback time. They are usually measured in bits by seconds. Depending on the amount of bits processed, the value can be given in kilobit per second (kbps or kbit/s), megabit per second (mbps or mbit/s), gigabit per second (gbps or gbit/s) and so on.
As a rule of thumb, especially in files with lossy compression like MP3, it can be said that the audio quality is better the higher the audio bitrate is.