A saving throw (sometimes called a save) represents an attempt to resist an effect being forced upon your character such as a spell, a trap, a poison, a disease, or similar threat. You don’t normally decide to make a saving throw; you make one because you’re at risk of harm. Although you typically will not want to, you can always choose to fail a saving throw.
To make a saving throw, roll a d20 and add the appropriate ability modifier. For example, you use your Constitution modifier for a Constitution saving throw.
A saving throw can be modified by a situational bonus or penalty, and can be affected by advantage and disadvantage, as determined by the Narrator.
Each class gives proficiency in at least two saving throws. The, for example, is proficient in Constitution and Charisma saving throws. Proficiency in a saving throw lets you add your proficiency bonus to saving throws made using a particular ability score. Some monsters have saving throw proficiencies as well.
The Difficulty Class for a saving throw is determined by the effect that causes it. For example, the DC for a saving throw allowed by a spell is determined by the caster’s spellcasting ability and proficiency bonus.
The result of a successful or failed saving throw is also detailed in the effect that allows the save. Usually a successful save means that a creature suffers either no harm or reduced harm.