A saving throw represents a desperate attempt to resist a special attack, a trap, a poison, a disease, and similar threats. You make checks and attacks when you decide to take an action. You make saving throws in reaction to events that happen to you.
Saving throws can have additional modifiers from feats or be affected by advantage and disadvantage depending on the circumstances. Some bonuses and penalties apply to all saves made with an ability. Others apply only under certain circumstances.
When you make a saving throw, the effect you attempt to resist has a DC. Powerful effects have higher DCs, while weak ones have lower DCs.
If you fail the saving throw, you suffer the full force of the effect you attempted to resist. You are caught by an explosion and take damage, you might fall into a pit that opens beneath you, or a poison might sap your vitality.
If you succeed on the saving throw, you either avoid the effect or suffer a diminished version of it. You might take cover and suffer only partial damage from the explosion. As a pit opens, you might leap to solid ground. A poison might cause you to feel ill, but you are durable enough to shrug off some of its effects.
You can’t take 7, 10 or 18 on a saving throw.
Saving throws usually require no action, unless stated otherwise.