The resistance the blower pushes against in your duct system — HVAC's blood pressure. High static (undersized returns, choked filters, crushed flex duct) strains motors and starves rooms of airflow; it's measurable in minutes with the right ports.
Air-quality terms map to humidity control in muggy Chesapeake summers, filtration (MERV), ventilation for tighter renovated homes, and duct condition in houses that have seen decades of retrofits.