What is bitrate?
Bitrate is how much data is spent on each second of a file. It is the quality setting that most people never look at, and often matters more than resolution.
Resolution is the frame, bitrate is the detail
Resolution tells you how many pixels there are. Bitrate tells you how much information is used to describe them. A 1080p video at a very low bitrate looks blocky and smeared, despite having all those pixels.
This is why two files at the same resolution can look completely different. The one with more data per second holds up during fast motion; the other falls apart the moment anything moves.
Audio bitrate, in numbers you can use
64 and 96 kbps are fine for speech and keep long recordings small. 128 kbps is acceptable for casual music listening. 192 kbps is the practical sweet spot — most listeners cannot separate it from higher settings on normal headphones. 320 kbps is the MP3 maximum.
The catch is that bitrate cannot recover what was already thrown away. Re-encoding a 128 kbps source at 320 produces a file two and a half times the size containing exactly the same sound.
Variable versus constant
Constant bitrate spends the same data on every second, whether it is silence or a full orchestra. Variable bitrate spends more where the content is complex and less where it is simple.
Variable generally gives better quality for the same file size, which is why most modern encoders use it by default.