The Sixth Age
The worlds (plural) refers to the three material planes;
The Feywild. As it is hard for most of the races not native to the Feywild to travel to and from that plane, it avoided much of the fighting of the Age of Discontent.
The Shadowfell. Being mostly uninhabited, and due to the fact that the walls to this Plane were thin due to the constant flow of the departed souls across them, much of the fighting during the Fourth Age took place in the Shadowfell, leaving it badly damage, barren and desolate.
The World or the Middle World, where most of The Sixth Age takes place. Largely destroyed by the races at war, the Middle World has slowly been restored. However, with magic disappearing from the hearts and minds of it inhabitants, it is unlikely it will ever be fully repaired.