Bitrate is how many bits per second a compressed audio stream consumes, quoted in kbps, and it is the master dial trading quality against size and bandwidth. Rough landmarks: 128 kbps AAC stereo is good for video, 256 kbps is near-transparent, Opus voice runs 16-32 kbps, and 5.1 E-AC-3 sits around 384-768 kbps. The relationship to quality is codec-dependent and non-linear — a better codec sounds the same at a lower rate — and it interacts with sample rate and channel count. Choosing a bitrate is really choosing a point on each codec's quality curve for your audience's networks and devices, which is what adaptive streaming automates with a ladder of rates.
