The zone air outputs are described here. Essentially, the Zone Air Temperature is the well-stirred zone air temperature reported at each system time step. An EnergyPlus simulation can iterate multiple system time steps timesteps within a single zone time step, timestep, so the average of the system time step air temperatures is reported as the 'Zone Mean Air Temperature'.

Also, to be slightly nit-picky, these outputs are resulting from the EnergyPlus simulation; OpenStudio is an interface that supports EnergyPlus simulations.