Bitrate
How many bits per second an audio or video stream consumes — the single most important dial for quality against file size.
At constant bitrate every second gets the same amount of data, which is predictable for streaming but wasteful: a static shot needs far less than an action scene. Variable bitrate redistributes the budget where it does good and gives better quality at the same file size.
The number cannot be compared across codecs. AV1 at 3 Mbit/s looks better than H.264 at the same figure, because newer codecs simply do more with each bit. Adaptive streaming exploits this by keeping several versions and switching on the fly as the network degrades — that is what happens when YouTube suddenly goes blurry.