An enema, where fluid is inserted into the bowel via the rectum, is a very useful tool in a doctor or a nurse's toolkit.
It's also often the butt of many, many a joke. (Forgive me for that one. I couldn't help myself).
In modern medicine enemas are most frequently used to help with constipation, clear the bowel out prior to a surgical procedure such as a colonoscopy or to help in imaging, for example in a barium enema.
However it may surprise you to know that history of human's sticking fluid up their rectums is a long and, well, somewhat obsessed one.