Hand size determines how many cards a player can keep when the cleanup step arrives, with the normal maximum being seven. Some cards remove that limit entirely, increase it, or reduce how many cards an opponent is allowed to retain.
Praetor's Counsel gives you no maximum hand size while returning your graveyard to your hand, while Folio of Fancies can alter everyone's hand size and punish players for holding too many cards. Effects that generate large hands become considerably stronger when you don't have to discard the excess.