Based on my understanding, if something different happens in one timeline, it branches off into a new timeline (alternate) which will create a new (alternate) universe. So the multiverse is basically different timelines that branched off from the original timeline.
I've been hearing a lot about "Sacred Timeline", so I would guess this would be the Earth-199999 or the MCU? I'm sure a lot of this is explained in the Loki series, I just need to wait a few more days before I can finally binge watch it.