Each person has two eustachian tubes. They are actually tubes (that is, they are empty spaces) running from your middle ear behind your eardrums down to the back of your throat.
The three tiny bones of your middle ear transmit the vibrations of your eardrum across the empty space at the top of your eustachian tubes to the cochlea
They allow you to equalize the pressure on both sides of your eardrums. If they get plugged up, then pressure changes won't be equalized and may cause pain as the air pressure pushes on one side of your eardrum more than on the other side.
You may have experienced the effects of a partial blockage of your eustachian tubes when you go up in the mountains (or in an airplane) and your ears "pop". Chewing gum can cause a partial blockage of your eustachian tubes to be opened temporarily ... that's why people sometimes chew gum on airplanes.
The eustachian tubes are also susceptible to retained fluid. When your eustachian tubes are blocked or filled with fluid, your hearing is impaired and you may have painful infections in your ears.