Static pressure is the pressure created by everything else in the ductwork that resists the airflow from the fan. This can include anything from the ductwork itself to the various dampers, coils, and other components installed in the system.
Static pressure is measured in “inches of water column” or simply “in. wc”.
When static pressure increases, the HVAC system must work harder to overcome it. This means the system must increase the volume of air it is pushing and which also increases the energy being used.