This checks for lands you control with the land type Mountain or Forest, not for lands named Mountain or Forest. The lands it checks for don't have to be basic lands. For example, if you control Temple Garden (a nonbasic land with the land types Forest and Plains), Rootbound Crag will enter untapped.

As this is entering, it checks for lands that are already on the battlefield. It won't see lands that are entering at the same time (due to Warp World, for example).