A hauberk was part of the suit of armor worn by knights. Made of strong chain mail, a knight's hauberk helped protect the upper half of his body during a battle.
Chain mail, which is basically metal fabric, was a relatively lightweight part of a medieval knight's armor. The word hauberk comes from the Frankish halsberg, "neck-cover," a small section of chain mail that protected only a knight's neck and shoulders. The hauberk expanded on this, covering more of the body from blows, sometimes extending from neck to knees.