A cochlear implant is an implantable device that stimulates that auditory nerve of people with severe-profound sensorineural hearing loss.
My basic* elevator speech explaining cochlear implants is as follows:
Inside your ear there is a spiral-shaped organ called the cochlea. The cochlea contains many little hairs that move when sound enters your ear and respond to different ptches, organized from low to high, like a piano keyboard. Different pitched sounds vibrate different sections of the hair cells. Those hair cells then send signals to the auditory nerve which goes to your brain and gives you the sensation of “hearing.” In a person with sensorineural hearing loss, those hair cells are not working. For some people, a hearing aid pushes enough sound into the air to stimulate the hair cells, but other people with more severe hearing loss can benefit from a cochlear implant. The cochlear implant inserts a tiny wire with electrodes into the cochlea to take the place of the damaged hair cells. Sound goes into the external cochlear implant processor (which looks kind of like a hearing aid), which translates the sound into electrical signals. The signals travel up the wire, where they are communicated to the internal implant via a magnet connecting the inner and outer parts. The electrically coded sound stimulates electrodes corresponding to the pitches of the sound, stimulating the auditory nerve and allowing a person who is deaf to hear.
*This is obviously a very, very simplified explanation, with some technical details condensed or glossed over, but it’s a quick and easy way to explain CIs to a person with little or no prior knowledge of hearing loss in a short period of time.