Plain English Meaning
If you fly into air where the pressure is lower than where you started, and you don't update your altimeter, the altimeter will tell you you're higher than you really are. You're actually flying lower than the number on the dial.
Full Definition
An altimeter error condition in which an aircraft flies from an area of higher atmospheric pressure into an area of lower atmospheric pressure without resetting the altimeter. Because the altimeter senses the lower pressure as a higher altitude, the instrument over-reads, and the aircraft is actually lower than indicated.
Context Anchor
Seen in basic aerodynamics when learning how a wing produces lift.