Sheathing is the proces of applying sheets of plywood (or OSB in this case) to the outside walls and roof of a structure. Sheathing doesn't protect a structure from the elements. This is the job of siding and roofing. Sheathing gives the structure strength by tying all of the framing members together. Before the Sheathing was installed when I stood on the loft of the building the whole thing swayed slightly. With the sheathing installed the whole thing is "solid as a rock."