What is "edain"

In the fiction of J. R. R. Tolkien, the Edain were men (humans) who made their way into Beleriand in the First Age, and were friendly to the Elves.

The Sindarin word Edain (the second syllable is pronounced as English dine rather than English dane; the stress falls on the first syllable), singular Adan (Quenya Atani, Atan) literally meant Second People, and originally referred to all Men, but later it only applied to the Men of Beleriand and their descendants. The Quenya term Atani kept its old meaning.

They were divided in three large houses, or tribes:

1. The House of Bëor: they were dark-haired and stoutly built, and most resembled the Noldor of all Elves. They were first discovered by Finrod Felagund, Lord of Nargothrond, and under his guidance later made their way to the lands of the Noldorin lord Amrod, in a place later known as Estolad, the Encampment. They remained loyal to the House of Finarfin, and later settled in the lands of Dorthonion.
2. The Second House, later known as the Haladin or the House of Haleth. They were a reclusive folk, dark-haired but smaller in stature than the Bëorians. They kept separate from the other Men, and later received permission to settle in the forest of Brethil, part of Doriath. They were a tribal people and were reclusive, and mostly kept out of the wars.
3. The House of Marach, later best known as the House of Hador. They were tall and golden-haired, and most resembled the Vanyar of all Elves. They were very numerous. They later settled in Hithlum by way of Estolad. They were loyal to the House of Fingolfin.

The Bëorians and Marachians shared a common tongue and were known to each other before settling in Beleriand. The tongue of the Haladin was alien to them.

The House of Bëor was nearly wiped out by Morgoth, and the remainder of its people merged with the Hadorians to become the Númenóreans. It would seem that the Haladin of Beleriand were completely wiped out, or at least disappeared as a separate people.

The Half-elven Elros was heir to the lordship of all three houses of the Edain, and chose to become one of the Edain. He became the first King of Númenor.

When the Númenóreans returned to Middle-earth in the Second Age, they encountered many Men who were obviously related to the Atani: they classified these Men as Middle Men, and established friendly relations with them. Examples are the Rohirrim, the Men of Dale, and the Breelanders.

Other Men, such as the Dunlendings, were not recognised as Middle Men because they were related to the Haladin rather than Bëorians or Marachians, and they were hostile to Númenor.

A fourth kind of Men came with the Second House, and called themselves Drughu. This name was adopted in Sindarin as Drúedain or Drû-folk. They were a strange people, living with the Haladin (and possibly related to them) in the forest of Brethil, some even apparently made it to Númenor, but they died out or had left before the Akallabêth. In the Third Age, their kin were known as the Woses of Drúadan forest.