Who is Men from The Silmarillion and what is their importance?

Men are the Younger Children of Iluvatar or the Secondborn. They are also referred to as Atani, Easterlings, the Second People, Engwar, Firimar, the Followers, or the Aftercomers. The Swarthy Men enter Beleriand from the east after the Dagor Bragollach and fight on both sides of the battle in the Nirnaeth Arnoediad. Morgoth allows them to dwell in Hithlum where they oppress the remainder of Hador's people. The term Atani specifically refers to the Men who were known to the Sindar and Noldor as friends. Sauron and Melkor both prey on Men more than Elves. Men are more easily inclined to evil than Elves, and as a result, Iluvatar makes them mortal. Men and Elves ally themselves for a long time until the death of Elendir, after which Men are estranged from Elves and usurp the light of Middle-earth.