A saving throw is a resistance roll a character makes to endure or reduce harm when caught in a damaging effect such as poison, a spell, or a trap. You roll, and if you clear the set target number you avoid the effect or take only half. Unlike an active attack, it decides the luck of the one enduring an oncoming danger.
A single roll to merely hold out when the unavoidable arrives resembles the last resistance a human has before an indifferent world.