Permanent damage to hearing is a form of hearing impairment and is caused by physical damage to your hearing.
If you are exposed to sounds louder than 80 decibels, for example, then the vibrating hairs in the cochlea become broken. Vibrations that are too strong cause them to break, just like a tree is uprooted during a storm. Then you have incurred permanent damage to your hearing, as these hairs do no grown back.
In the case of permanent damage to hearing, always use hearing protection to prevent more severe damage or even deafness.