The Barrier is the mystical wall between The Waking and The Dreaming, supposedly erected by Solus herself to divide her previous creations from her continued experiments in creating new life. But The Barrier is much more than a magical membrane, or wall. It is a plane unto itself, with its own rules and laws. Some refer to The Barrier as the Plane of Mirrors, or the Plane of Doors, for it is easiest to enter through reflective surfaces, or through teleportation magics.
When arcane scholars talk about The Barrier they usually refer to it as the Plane of Mirrors. To them it isn’t clear that this plane does anything to hold The Waking and The Dreaming apart, or that both of those planes were ever together in the first place. What concerns such experts in magic theory are the uses the Plane of Mirrors can be put to when casting magic spells and rituals. Those uses include quick teleportation, rapid long-distance travel through shrunken mirror realms, and the creation of pocket planes and extra-dimensional spaces for storing large amounts of stuff in seemingly small spaces.
The mysterious changeling race is theorized to come from the Plane of Mirrors, and are probably not its only natives if this theory is accurate.