A cochlear implant is an electronic device that can restore hearing to people with certain types of hearing loss. Unlike a traditional hearing aid, which simply amplifies sound which is then heard in the normal way, a cochlear implant converts sound to electrical impulses which are then transmitted to the nerves that would normally carry the auditory signal to the brain.
For children who are born deaf, cochlear implants offer the ability to learn to hear normally. If hearing loss happens later in life, adapting to a cochlear implant may not be as successful. Because the signals transmitted to the brain are similar to, but not identical with, what the normal ear would produce, a formerly-hearing person who receives a cochlear implant will at first not be able to understand the signal. The brain has to be 'retrained' to correctly decode the incoming signal, a process that can take some time.
For people who are born deaf and grow to adulthood without hearing, adapting to a cochlear implant is even more of a challenge. During the time in which an infant's brain is learning to process auditory signals, the deaf infant's brain is learning other things, so that later, an auditory signal has no processing 'software' to deal with it and the portions of the brain usually used for dealing with sound are programmed to deal with other kinds of information. The earlier a deaf child is fitted with a cochlear implant, the more successful their hearing adaptation will be, since they can learn to process incoming sound during the normal infant learning phase.
Interestingly, there is some objection to the use of cochlear implants in a portion of the deaf community, the signing community, or those who communicate by sign language alone and consider themselves a cultural group. They feel that their deafness isn't a disability and that by the use of cochlear implants in infants who are born deaf, the larger society is effectively killing their community by preventing others from having to deal with deafness. They claim that the infants aren't able to give informed consent over which community -- the hearing or signing community -- they wish to be members of. And of course, if left until a child was old enough to make that decision for themselves, adapting to a cochlear implant would be significantly harder and less successful.