Alfheim is one of the Nine Realms and the home of the Light Elves.
During the Convergence in the distant past, the Nine Realms aligned and the borders between worlds became blurred. One of the worlds in the Convergence was Alfheim.
When Thor was exiled to Earth in order to learn humility, he did not know where he was. After being hit by a van carrying Jane Foster, Darcy Lewis, and Erik Selvig, he asked whether he was in Alfheim or not.
After the destruction of the Bifrost, the Nine Realms, including Alfheim, were left vulnerable to attack without Asgardian protection.
When the Convergence occurred again in present day, Alfheim once again became aligned with the other Realms.