Why Do Your Ears Pop During a Flight?

Cabin pressure changes as an aircraft climbs and descends, but the middle ear is an almost enclosed air-filled space behind the eardrum. Its pressure cannot always change at the same rate. A pressure difference across the eardrum can make it bend slightly, producing fullness, muffled hearing or discomfort.

The middle ear connects to the back of the nose and throat through the Eustachian tube. This passage is normally closed but opens briefly when we swallow or yawn, allowing air to move and pressure to equalise. The familiar pop or click often occurs when the tube opens suddenly and the pressures on the two sides of the eardrum become similar again. Nothing has exploded; the pressure balance has simply been restored quickly.

The FAA explains that during a climb, expanding middle-ear air can usually escape outwards more readily. During descent, cabin pressure rises and air must enter the middle ear, but the Eustachian tube may not open as easily, so the sensation is often stronger before landing. Congestion from a cold or allergy can slow equalisation. Swallowing, chewing or yawning can help because these actions engage muscles that open the tube. Persistent pain or hearing change falls outside this ordinary mechanism and needs professional assessment.

https://www.faa.gov/air_traffic/publications/atpubs/aim_html/chap8_section_1.html
https://www.nlm.nih.gov/medlineplus/ency/article/001064.htm