The Multiverse is ultimately used as a plot device to keep the headcanons and multiple Tracy rp accounts all legitimate and able to be accounted for.
Despite the multiple accounts being open simultaneously almost since the beginning of the Tooth Fairy Fandom on Tumblr, The Multiverse was not always open. As ideas for the Multiverse concept were solidified, it happened that several months ago, Willow cast an incorrrect spell that backfired and linked the worlds. Fairy, being the location of the highest concentration of magic in Fairy Land, became the hub for the gates that would connect the worlds.
The Gates that connect the worlds are in constant, always present positions save for the gate to the world of origin (ie, the world you find yourself in will not contain a Gate to the world you find yourself in). The Gates are discreet and often built into walls so that you don't just go meandering into another world - hence why most of the world inhabitants aren't aware. The only exception to this is for a short period of time after a Gate opens, when it is still trying to settle into a location. During this short time, the Gate is liable to jump around, making it very easy to accidentally move from one world to another.
There are currently eight open gates and eight corresponding worlds consisting of
Tracy and Sam
Tracy and Rae
Willow and Elizabeth
Article to be finished later :)