Barrier

Barrier is a defensive summoner spell that gives your champion a large shield for two seconds. Unlike Heal, it only affects you and provides no movement speed, but the shield amount is bigger and applies instantly.

Players take Barrier when they expect to absorb a clear burst of incoming damage, such as an enemy assassin's combo, since a well-timed shield can negate the kill entirely. It is popular on mid-lane mages and carries who win once they survive the first wave of damage. Because it is pure mitigation, Barrier is not weakened by healing-reduction effects the way Heal is.