Plain English Meaning
When you climb or descend, the air inside your ears and sinuses needs to move in and out to match the changing pressure. If you have a cold or stuffy sinuses, those passages get blocked and the trapped air can't equalize, which causes pain, pressure, and sometimes hearing trouble — especially on the way down.
Full Definition
Pain, pressure, or temporary hearing impairment caused when air trapped inside the middle ear or the sinuses cannot equalize with changing cabin pressure during climb or descent. As outside pressure drops in a climb, trapped air expands and usually vents out easily; as pressure rises in a descent, outside air must flow back in through the Eustachian tube (for the middle ear) and the small sinus openings. If those passages are swollen or blocked — typically by a cold, allergies, or sinus congestion — the pressure cannot equalize, producing discomfort, sharp pain, possible eardrum injury, vertigo, or temporary hearing loss.