Refer to figure.
The Eustachian tube equalizes the air pressure between the throat and the middle ear. This ensures that the same pressure applies in both sides of the eardrum (outer and middle ear side).

The eardrum is a thin, flexible membrane that separates the outer ear from the middle ear. Air that comes to the eardrum from the outer ear side is environmental air and immediately adapts to pressure changes as we climb or descent. The air that exists from the other side of the eardrum is 'trapped' inside the middle ear and does not change pressure automatically. However, there is a tube that connects the middle ear to the back of the throat called the Eustachian tube. This tube is normally closed, and opens when necessary to help the middle ear adapt to any changes in atmospheric pressure.

Air that enters through our throat as we breath is environmental air that adapts to pressure changes as we climb or descent. When the Eustachian tube opens, it allows the air from the throat to enter the middle ear and causes the air of the middle ear to adapt to any pressure changes. This process ensures that the same pressure exists at both sides of the eardrum.