Apps randomly turned on in “Apps using iCloud”?

I don’t and never intend to use iCloud. But once in a while, I have to go to the “Apps using iCloud” settings to inspect, because sometimes a random app would be turned on.

(1) What does it mean to have an app turned on? I tried to log in to my iCloud account and could never figure out where those apps might be stored. (2) Is there a way to turn it off completely?