Well, in Florial's case I would say he has a high upside IF he can develop the ability to lay off high fastballs out of the zone and foul off high fastballs in the zone. And to know the difference. His upside is based on both power and a good eye for the strike zone. Just power alone doesn't cut it.
Florial's walk percentage is the third highest on the Guardians behind Fry and Bo, and it's not like pitchers are intimidated and pitching around him. His out-of-zone swing percentage is 30%, which is pretty good for a young player. It's better than Josh, Jose, and Gimenez so far this year.
For a guy in his first year on a new team trying to earn playing time and getting sporadic opportunities, I expected him to be a lot more aggressive and chase more bad pitches. He's a pretty patient hitter, which surprised me.
These are small samples, of course, but I think they're encouraging.
I'd say his "high ceiling" is based on a combination of power, plate discipline, and a good eye for the zone. Lay off the high cheese and hunt low pitches you can pull, Estevan.