Plain English Meaning
If the air pressure underneath the aircraft is higher than what the altimeter is set to, the altimeter shows the aircraft as being lower than it really is. The aircraft is actually higher than the instrument suggests.
Full Definition
An altimeter error condition in which the actual atmospheric pressure at the aircraft's location is higher than the pressure setting in the altimeter, causing the altimeter to indicate a lower altitude than the aircraft is actually flying. This typically occurs when an aircraft flies from an area of lower pressure into an area of higher pressure without updating the altimeter setting.
Context Anchor
Seen in basic aerodynamics explanations of how a wing produces lift.
Derivation
Pressure comes from a Latin word meaning “to press.” In this term, it means pressing against a . “Below” points to the lower side of , where that pressing is greater than it is above the wing.