A saving throw is best described as a roll one makes for their character in order to avoid some kind of effect or hazard. Compare the saving throw with Defy Danger which is literally described as «When you act despite an imminent threat or suffer a calamity, say how you deal with it and roll.» Compare this also with Acting Under Fire which is described as «Call for this move whenever someone does something requiring unusual discipline, resolve, endurance or care.» In the environment of a game, one of these rules is just a mechanic that allows the GM to call for a roll to avoid some type of hazard and the other is the exact same thing!