It is the average of all of the energy in a region in space.  If you took that average, there would be groups of random particles that exist at one moment in time.  Then that area would have an energy level greater than zero.  It is just another way of looking at it to get around the whole argument that random particle pairs don't actually add energy to a system.  If the system actually is shown to have more energy, then that energy would have to come from somewhere, is the argument.  If it come