Armor is one of the most basic ways for your character to defend themselves from harm. There are two kinds of armor - light armor, which is flexible and doesn't encumber movement, and heavy armor, which offers better protection but at the cost of mobility.
Your Armor defense is equal to armor's base defense + your Proficiency with that armor type. Your light armor proficiency is based on your Dexterity, while your heavy armor proficiency is based on your Constitution. Most physical attacks, including basic melee and ranged attacks, target your Armor.