The eardrum can have a hole or defect present from a variety of reasons including trauma (cotton swab, hair pin, slap to the ear, diving accident, etc). Otherwise, a severe acute infection of the middle ear or a long-standing problem with the eustachian tube can result in a hole (perforation) of the eardrum.
The holes in the eardrums (tympanic membrane perforations) can be described as central or marginal. Central TM perforations are usually safe except that they can cause some hearing loss and can predispose one to middle ear contamination from dirty water and, therefore, ear infections.
Marginal eardrum perforations are less safe. Since they are present at the edge or margin of the tympanic membrane, the marginal perforations can give rise to skin invasion of the middle ear space from the ear canal skin. Skin tissue does not belong in the middle ear and a chronic, even life-long ear infection is possible with a marginal perforation.
Thankfully, both central and marginal tympanic membrane perforations are amenable to surgical correction.