It isn't what is being covered, but why. See the mask on one guys face... see the masks hanging around the other guys necks? I will bet they are about to saw some concrete with a really dusty circular saw.
As the rockwork on the interior is well along, some of it hasn't been painted yet and some of it has. They don't need all that white concrete dust in there. That is what the plywood is there for, (even though security is hyper tight on the building from backstage). I guess they don't trust their employees back there because there are at least three large doors back there that are often open for air.