The eardrum is a thin membrane that helps to transmit sound. Air pressure is exerted on both sides of the eardrum; with the surrounding atmosphere pressure pushing it inwards while air being delivered via a tube between the back of your nose and the eardrum pushes it outwards. This narrow tube is called the Eustachian tube, when you swallow the tube opens and a small bubble of air is able to move causing a ‘pop’.
Rapid altitude changes in plnaes make the ‘pop’ much more noticeable due to bigger differences in pressure. Air pressure decreases as a plane ascends; hence air must exit the Eustachian tubes to equalise these pressures, again causing a ‘pop’. Conversely, as a plane descends, the air pressure starts to increase; therefore the Eustacahian tubes must open to allow through more air in order to equalise the pressure again, causing another ‘pop’. So there’s no need to worry, although popping ears are uncomfortable they’re part of a completely natural process.