In order to answer this question completely, you must first understand some normal anatomy of the ear. The external auditory canal (ear canal) extends internal to the level of the tympanic membrane (ear drum). Then everything medial to the tympanic membrane to the level of the cochlea is called the middle ear. There are three bones of hearing (malleus, incus, stapes) that are attached to the internal aspect of the tympanic membrane and function to transmit the vibrations of the TM (tympanic membrane) to the cochlea which converts the vibratory signal into an electrical signal which is then sent to your brain. This is how your brain interprets sound.
In order for the entire process to work properly (in particular the middle ear), there must be air within the middle ear. It gets there through the eustacian tube. The eustacian tube brings air up from the back of the nose to the ear. It this tube isn't functioning properly, it can lead to negative pressure within the middle ear that can suck fluid from the surrounding tissues and cause a middle ear effusion. This fluid can then repeatedly get infected. In children vs adults, there are theories about the angle of the eustacian tube, and functionality of it which lead to more frequent ear infections. I hope this helps.