After the Valar banished Morgoth, the men of Westernesse fell under the sway of Sauron, who at the time still could appear good. He convinced them that death was the greatest injustice of all time, and that they should sail away and take the Undying Lands by force.
Foolishly heeding this advice, the king of Westernesse and his advisors mounted a force and sailed into the West. They were utterly destroyed, and in retaliation for their pride the island kingdom of Westernesse was destroyed and sank beneath the waves. Only a few escaped; among them Elendil and his son Isildur, who had not listened to Sauron's counsel.
It was they who founded the Northern and Southern Kingdoms of men in Middle Earth.
The men of Númenor seem to represent the virtues of knowledge and wisdom, but also provide a caution against the pride that comes with such virtues.
