The Mirror Realm is a secret dimension that is behind every mirror in the world. It can be accessed only through magical spells, from powerful magic wielders such as Milton or Giles Grimm, or, though not anymore, through an ancient artifact called the booking glass, which is currently shattered.
Presumably, the Mirror Realm had existed ever since the creation of the first mirror, though always deserted.
That is until one day, the Evil Queen was banished to the Mirror Realm by Giles and Milton Grimm and Baba Yaga, for trying to take over Ever After and for messing with destinies and unleashing a powerful curse over Wonderland.
For nine years the Evil Queen was kept prisoner in the Mirror Realm, with access to every mirror in the world, able to see what went on without her through the looking glasses. She was able to escape once, in "Dragon Games", but was imprisoned again and currently still resides within the Mirror Realm.
The Mirror Realm is a dark dimension, the only light that shines is through the looking glasses that are randomly placed around the dimension, though these aren't quite like mirrors, more like portals that allow vision to the outside world, wherever the respective mirror is positioned. Any object can be captured in it, through the booking glass, and whoever is inside can still interact with the outside world, though with limits.