Support (aka Control) is one of the four roles that Heroes and Angels can play. The others being DPS, Healer, and Tank. Good team building typically requires Heroes and/or Angels that play each of these roles.
Support characters typically bring skills which help their team in indirect ways. These skills may improve (buff) their team's abilities, reduce (debuff) their opponent's abilities or offer other benefits such as control skills which can stun an enemy, making them unable to attack for a period of time.