Basically, the locked forums are prizes. Once the site (or specific people) meet certain requirements, they will be unlocked. As an example, one of the den areas will randomly be unlocked once the forum has a certain number of members. Others can be unlocked with various other points reached.

Locked Areas
Northern Tundra
-Northern Dens
-Rabbit Tundra
-White Lake

Eastern Mountains
-Cliffside Dens

Southern Forests
-Blossom Meadow
-Secret Stream
-Wooded Dens

Western Desert
-Cactus Oasis
-Prey Run
-Riverbed Dens