Short answers, pulled from the story.
Men in Middle-earth are the mortal race in J. R. R. Tolkien's fictional world, created as the 'younger children' of the One God Iluvatar, born after the Elves. Unlike the immortal Elves, when Men die their souls depart to a realm unknown even to the godlike Valar. They are one of the four free peoples of Middle-earth, alongside Elves, Dwarves, and Ents.
Tolkien stated that the core theme of The Lord of the Rings was death and the human desire to escape it. The theme appears throughout the work but is sharpest in the appendix 'The Tale of Aragorn and Arwen,' where the immortal Elf Arwen chooses to become mortal to marry the Man Aragorn.
The Men of Numenor were destroyed after Sauron tempted them to attack Valinor, falsely claiming that immortality awaited anyone who arrived there. The island was drowned beneath the ocean in an Atlantis-like catastrophe, and the world was made round. Faithful survivors under their leader Elendil sailed to Middle-earth and founded the kingdoms of Arnor and Gondor.
The Rohirrim, with their wooden mead-halls and horse culture, correspond to the Anglo-Saxons and the Goths. Gondor, with its stone cities, literature, and music, corresponds to the Byzantine Empire and, in other features, Ancient Egypt.
Aragorn and Boromir are the only two Men in the Fellowship of the Ring, and their responses to the Ring's temptation are opposite. Boromir tries to seize the Ring from Frodo at Parth Galen, repents, and dies defending the Hobbits; Aragorn resists at every turn and is eventually crowned king.
Scholars including David Ibata (writing in The Chicago Tribune), Margaret Sinex, and Zakarya Anwar have noted that the 'good' Men of Middle-earth are fair-skinned and live in the West, while adversaries are dark-skinned or 'slant-eyed' and live in the East and South. The scholar John Magoun describes this arrangement as a 'fully expressed moral geography.' Others note that Tolkien was strongly anti-racist in his actual life.