For the adult version of the Wiki explanation (edited for adults-only):
In the surreal, utopian Unimatrix Zero, Borg of various species from throughout the massive geography of the Collective exist as their individual, unassimilated selves and interact with one another as such. While out of regeneration, they revert to normal drones and have no memory of their time spent together there (since it was all a dream, right?). The Borg Queen knows about Unimatrix Zero, considering it a disease, and destroys drones discovered capable of visiting it. However, the process of detecting affected drones is very time consuming, and she is eager to find a faster method of finding and deactivating them. Those who are detected are deactivated, dismembered, and studied in an effort to speed up the search.
The description is uncanny, after a fashion...
So you may anticipate that I'll be warping in to a forum near you...