The fictional metal used to make Bilbo’s and Frodo’s famous mail shirt in Tolkien’s legendarium was mithril. In The Lord of the Rings, Bilbo gives Frodo a coat of mithril mail that he received during the events of The Hobbit. The shirt is exceptionally light and strong, making it valuable protection for its wearer.
Mithril is described as a rare metal found especially in Moria, whose Elvish name is Khazad-dûm. The Dwarves mined it extensively, and its value increased after the awakening of the Balrog drove them from the city. Tolkien also links mithril with the older substance associated with the legendary Silmarils in later lore, though the relationships are presented through layered mythic history.
Mithril is not the same as galvorn, another fictional metal mentioned in Tolkien’s wider writings. Nor is the shirt made from ordinary steel. Its importance becomes clear when Frodo survives blows that would have killed an unprotected traveller.