An enema describes any treatment or diagnostic procedure where a fluid is introduced into the rectum through the anus. It's most common use is to relieve constipation by loosening stool. It is also used in preparation of any procedure to treat or examine the large intestine or rectum. It's most common diagnostic use is as part of a barium enema.
More rarely, it is used to introduce, food, water or medication to a patient, including rehydration when intravenous fluids are not an option.