From what I understand, it's where the animals from the old park are still living. Masrani Corp just set up a 'safe zone' in the middle. Not entirely sure why, but hopefully that'll become clear.
Compared with the
best JP map available, it looks like the restricted zone starts slightly north of the old visitors' centre and main gate; the only film locations within it are the jeep tour paddocks.
Super spoiler; seriously
But I dimly remember hearing that one JW location will be the ruins of the old VC.