Magical Items: Magic Armor
From the manual:
Is this also valid if the character has no-armor?
Viewing post in RPG novice list of questions and clarifications
To answer this and your next question at the same time: the bonus of magic armor is added to the Armor value. So if you wear a magical set of +2 Light Armor your total armor would be 5 Armor (3 from the Light Armor and +2 because it is magical. So being magical just increases the amount of Armor points a set of armor has, beyond this it just works like normal Armor and is reduced when taking damage.
A magical +2 Light Armor is better than mundane Medium Armor (even though both grant the wearer 5 Armor in total) because the magical Light Armor only takes up 1 inventory slot while the non-magical Medium Armor takes up 2.