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 within a single zone time step, 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.

Search harder in the Input/Ouput reference guide. Chances are it's covered in there.

Quoting directly from the Input/output reference guide:

Zone Mean Air Temperature [C]

From the code definition, the zone mean air temperature is the average temperature of the air temperatures at the system timestep. Remember that the zone heat balance represents a “well stirred” model for a zone, therefore there is only one mean air temperature to represent the air temperature for the zone.

Zone Air Temperature [C]

This is very similar to the mean air temperature in the last field. The “well stirred” model for the zone is the basis, but this temperature is also available at the “detailed” system timestep.