Bitrate is the amount of data a video uses per second, written in kilobits or megabits per second — kbps and Mbps. It is the only one of the four file-size levers that relates to size by arithmetic rather than by judgement:
file size (MB) = bitrate (kbps) x duration (s) / 8192
That equation runs both ways, which is the useful part. If a platform caps you at 20 MB and your clip is 90 seconds long, you have about 1,820 kbps to spend across video and audio combined, and no setting anywhere will change that. A tool that promises to hit a target size is solving this equation; a tool that asks you to pick a quality level is not.
What bitrate does not tell you is whether the result looks acceptable. That depends on what you filmed. In our own measurements on 1080p footage, a screen recording scored 95.5 VMAF at 1,244 kbps, while camera footage needed 5,705 kbps to reach a comparable 92.5 — more than four times the data for a slightly worse score. Flat, static frames compress cheaply; grass, water, skin and motion do not.
So bitrate is best understood as a budget rather than a quality setting. Work out what the destination allows, divide by the duration, subtract the audio track, and see which resolution that leaves room for. Every platform page on this site shows that division worked out across a range of clip lengths.