Chain mail is the protective material that knights wear as part of a suit of armor. Made from small circles of metal, chain mail defends against slashing swords (not so much against fire-breathing dragons).
Chain mail is a flexible mesh material — basically a metal fabric. It's effective at protecting knights from cutting and slashing weapons, but somewhat less protective against sharp thrusting blades. In the Middle Ages, the shirt known as a hauberk was made of chain mail, and it provided a relatively lightweight alternative to hard, heavy plate armor. Sometimes chain mail is just called mail, from the Old French maille, "mesh."