Your Armor Class (AC) measures how well you can defend against attacks. When a creature attacks you, your Armor Class is the DC for that Attack roll.

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 for the specific type of armor you’re wearing. If you’re not wearing armor, use your Proficiency in Unarmored Defense.