Crowd control or CC is an RPG term for killing or managing large amounts of enemies at once. It can also mean to taking care of fewer, like "more than one".
Examples
Spells or skills that affect (damaging or just occupying) many enemies at once are called crowd control skills/spells. Such skills can be devastating, multi-target attacks, or more subtle techniques that draw all of the enemies to a single point, freeze or slow them, or somehow control their AI.