Unarmored

Without armor or a shield your AC is 10 + Dexterity Modifier. If you wear armor, you only get the Armor Class from the armor.

For example, Grogus the half-orc has +2 Dexterity modifier. When unarmored, she has AC 12. If she dons a Heavy Splint armor with AC 17, her Armor Class is 17.

Wearing no armor does not grant any particular benefits, unless the wearer has a feature that requires them to be unarmored.

For example, the Barbarian and Monk have an Unarmored Defense feature that grant them additional AC while not wearing any armor.