It means he'll only come out if he's in foul trouble.
I'd have the upperclassman backup both spots versus a tough opponent. Let the freshman play otherwise.
I never set a player at 'mopup', but I believe that if one of the other two players at that position gets hurt or in foul trouble, the other one will pick up extra minutes before the mopup would enter the game.
If you use the setting 'play backups more in a blowout', then I think the mopup will get more minutes as well