The eustachian tubes connect the middle ears camera.gif to the back of the throat. The tubes help the ears drain fluid. They also keep air pressure in the ears at the right level.
When you swallow or yawn, the tubes open briefly to let air in to make the pressure in the middle ears equal to the pressure outside of the ears. Sometimes fluid or negative pressure gets stuck in the middle ear. The pressure outside the ear gets too high. This causes ear pain and sometimes trouble hearing.