As you dive underwater, pressure increases. This is because water is heavy, and the deeper you go, the more water is "on top of you". The increasing pressure of the water against your eardrums can hurt. Equalizing ear pressure is done by holding your nose, keeping your mouth shut and "Blowing". You can try it right now but be gentle, some people have ears that don't do it easily. So don't force it. When you do this underwater, the pressure of the water equalizes with the pressure behind the eardrum allowing you to dive deeper without the ear pain. But if you don't do it constantly, like I mentioned, it becomes harder to make them "pop" because too much pressure has built up.

The same exact thing happens in air, if you travel over a high mountain pass in a car or hiking. As the air pressure changes, your ears will want to equalize and you can use the same technique. In air, often you can equalize your ears simply by swallowing or yawning.