Defenses

Attack rolls are compared to a special DC called Armor Class (AC), which measures how hard it is for your foes to hit you with Strikes, spell attack rolls, and other attacks. Just like for any other check and DC, the result of an attack roll must meet or exceed your AC to be successful, which allows your foe to deal damage to you. AC is calculated using the following formula.
Armor Class = 10 + Dexterity modifier (up to your armor's Dex Cap) + proficiency bonus + armor's item bonus to AC + other bonuses + penalties
Use your proficiency bonus for the category (light, medium, or heavy) or the specific type of armor you're wearing. If you're not wearing armor, use your proficiency in unarmored defense.
Armor Class can benefit from bonuses with a variety of sources, much like attack rolls. Armor itself grants an item bonus, so other item bonuses usually won't apply to your AC, but magic armor can increase the item bonus granted by your armor.
Penalties to AC come from situations and effects in much the same way bonuses do. Circumstance penalties come from unfavorable situations, and status penalties come from effects that impede your abilities or from broken armor. You take an item penalty when you wear shoddy armor.

There are three types of saving throws: Fortitude saves, Reflex saves, and Will saves. These are frequently called “saves”— it's the same thing. Saving throws measure your ability to shrug off harmful effects in the form of afflictions, damage, or conditions. You'll always add a proficiency bonus to each save. Your class might give a different proficiency to each save, but you'll be trained at minimum. Some circumstances and spells might give you circumstance or status bonuses to saves, and you might find a medpatch or other items that give an item bonus.
Fortitude saving throws allow you to reduce the effects of abilities and afflictions that can debilitate the body. They use your Constitution modifier and are calculated as shown in the formula below.
Fortitude save result = d20 roll + Constitution modifier + proficiency bonus + other bonuses + penalties
Reflex saving throws measure how well you can respond quickly to a situation and how gracefully you can avoid effects that have been thrown at you. They use your Dexterity modifier and are calculated as shown in the formula below.
Reflex save result = d20 roll + Dexterity modifier + proficiency bonus + other bonuses + penalties
Will saving throws measure how well you can resist attacks to your mind and spirit. They use your Wisdom modifier and are calculated as shown in the formula below.
Will save result = d20 roll + Wisdom modifier + proficiency bonus + other bonuses + penalties
Sometimes you'll need to know your DC for a given saving throw (such as a Grapple attempt requiring a roll against your Fortitude DC). Like any other DC derived from a modifier, the DC for a saving throw is 10 + the total modifier for that saving throw.
Most of the time, when you attempt a saving throw, you don't have to use your actions or your reaction. You don't even need to be able to act to attempt saving throws. However, in some special cases you might have to take an action to attempt a save. For instance, you can try to recover from the sickened condition by spending an action to attempt a Fortitude save.