Source: Contracting Business
In terms of HVAC systems, static pressure is the resistance against the air flow.
Static pressure can emanate from a variety of sources, including the HVAC’s own components, such as its ductwork and filters, among others.
In HVAC ductwork, the pressure is highest when the fan pushes the air, while it is at its lowest in the place where it enters the room (e.g., supply grille).
For the HVAC system to work effectively, the air throw must be greater than the static pressure, or the air will not flow through to the areas in need of cooling or heating. Furthermore, if there’s too much static pressure, your HVAC system will not work effectively.