Plain English Meaning
The aircraft keeps the inside air thicker than the outside air so people can breathe normally even when flying very high.
Full Definition
A system that pumps conditioned air into a sealed aircraft cabin to maintain an interior air pressure higher than the outside atmospheric pressure, allowing occupants to breathe normally and remain comfortable while the aircraft operates at high altitudes where the outside air is too thin to sustain them.
Context Anchor
You will see this term in discussions of turbine-powered airplanes, high-altitude flight, environmental systems, and abnormal procedures for loss of cabin pressure.