Mithril is the fictional metal that J.R.R. Tolkien’s character Bilbo brings from the Lonely Mountain.
In The Hobbit, Bilbo takes a coat of mithril mail from the treasure hoard in Erebor. Thorin gives the coat to him after the Dwarves reclaim the mountain. The lightweight, exceptionally strong metal is described as being more valuable than gold, and the mail later protects Bilbo during his adventures.
Mithril also becomes important in The Lord of the Rings. Bilbo gives the mail to Frodo, who wears it during the journey to Mordor. It saves Frodo from a fatal blow in the mines of Moria and is later recognized as an extraordinary treasure in Minas Tirith.
The answer is sometimes confused with galvorn, another fictional metal found in Tolkien’s wider mythology. Mithril is specifically the famous silver-like metal associated with Moria, Númenor, and the Dwarves of Erebor.