Enthalpy, as it related to HVAC, is the amount of energy (heat) the air can absorb or release. This calculated based on the air's temperature, saturation level (humidity), and the area's barometric pressure.
Why is enthalpy important for HVAC?
HVAC equipment with economizers can have an increased energy efficiency value if enthalpy is used for deciding when to and when to not activate an economizer cycle.