The Feywild is a plane of existence. It is essentially a mirror of the Natural World that amplifies the nature and life. The Feywild was home to the Elves, before they were recently forced out by the Positive Elementals that they shared the plane with.
The Feywild was originally a different plane, the Plane of Positive Energy, which housed the Positive Elementals. It changed over time to become the Feywild, and the Elves decided they wanted to live there.