“Armor Class” or “AC” is a numeric value that represents how hard it is to hit someone. This is a combination of how agile a character is and how much armor they have. When you make an attack roll against a target, you must meet or beat their AC to hit them. To find your AC you start with 10, add your Proficiency with your defense, using your Dexterity as the relevant modifier, and then add any other bonuses or penalties you might have. In other words:
AC = 10 + Defense Proficiency (Dex) + Armor Bonus + Shield Bonus + Other Bonuses
This is the score that determines how good you are at defending yourself. Your defense proficiency is determined by your class. This doesn’t always correlate with how well you can fight; a barbarian attacks recklessly while a paladin has a more measured response. Try balancing out your class choices to make sure you have a solid defense!
If you are unaware of an attacker or otherwise can’t meaningfully resist an attack against you. Unaware AC is your AC but is capped at 15 (if it is higher than 15, it is set to 15).
If you have fortified armor this cap is raised to 20.
Some creatures are unusually hard to hurt, or more resilient against a particular damage type. In these instances the damage they would take is reduced by a certain amount. What it applies to is spelled out after the word “Resist”. “Resist fire 5” means you reduce all fire damage by 5. If something says “Resist all”, or simply “Resist” without giving a damage type, that means it applies against all damage types.
Some characters gain “threshold”, a representation of their ability to power through damage up to a point. This means if you take the listed damage or less, you take no damage. If you take more damage than your threshold, you take all of it.
Sometimes a particular damage type is especially effective against a target. These weaknesses are referred to as “vulnerabilities”. When a creature takes damage they are vulnerable to, they take double the normal damage.