This checks for lands you control with the land type Forest or Plains, not for lands named Forest or Plains. The lands it checks for don't have to be basic lands. For example, if you control Hallowed Fountain (a nonbasic land with the land types Plains and Island), Sunpetal Grove 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).