Lenses and cameras are real objects and have manufacturing tolerances which are generally good, but you'll almost never end up with a lens or camera whose specs precisely match the design. No two lenses and camera will be absolutely identical.
A lot of the time (most of the time) these differences are too small to worry about in practical shooting (even professionally).
But sometimes you get e.g. lens and/or camera (or more typically the combination) have tolerances which mean the result is just too far outside what you need for that perfect shot.
So to compensate you can set microfocus adjustment.
There are a lot of complications to doing that properly and, IMO, most people who use it have no idea what they're doing. Just to diagnose the problem properly you have to be certain your technique doing the formal testing is spot on. I've seen a lot of amateurs make things worse trying to correct focus issues which are really general technique related and not this issue I've described.