"Fee simple" is just a term for a form of ownership of real property (e.g. land); the PA owns the real property (the land and the former buildings) while Mr. Silverstein's interest in the property is/was a leasehold interest.
If you own a piece of land, you probably hold title in "fee simple", too, which really just means that it's yours and nobody else's (subject to, for instance, a mortgage interest that might be held by a bank, or an easement of some sort, a leasehold interest of a tenant that you may have rented it to, etc., but the real property is
yours.)
Mr. Silverstein was required by the terms of the lease to insure the buildings, which is pretty standard in commercial leasing.