Light armor is a form of armor that requires the armor proficiency (light) feat to equip and use effectively, and is worn in the Armor slot. It provides only basic protection against physical attacks but doesn't hinder movement as much as heavier armors so it's popular among characters who rely on dexterity. The spellcasting techniques of Bards and warlocks is simple enough that they have Armored Caster and can avoid arcane spell failure from these armors.
This type of armor includes: Padded armor, Leather armor, Studded leather armor, and Chain shirt. Any medium armor made with mithral is also considered light armor.