From time to time, a spell or some other kind of hazard requires you to make a “saving throw.” A successful saving throw means that the character (or monster) avoids the threat or lessens its effect.
Each character class has a Saving Throw target number, which gets lower and lower as the character gains levels.
To make a saving throw, roll a d20. If the result is equal to or higher than the character’s saving throw target number, the saving throw succeeds.
Monsters can also make saving throws (a monster’s saving throw target number is listed in the monster’s description).
Dice rolls based on a characteristic are handled very slightly differently to the Luck-based saves detailed above.
If you are asked for a DEX roll or CON roll (for example), you roll a d20 as usual, but you get to add (or subtract) your appropriate characteristic modifier to the result.