Above is correct: the lower part of the intestines coming out of the anus. Many times it can correct itself without help. Sometimes it has to be pushed back in with the fingers. Once my son had to be put under anesthesia so that the doctors could push it back in.
It is a symptom of CF. Once diagnosed, however, and the patient is taking enzymes the prolapses SHOULD (but not in all cases) stop.
What causes it? The frequent, bulky, and abnormal stools over time can cause it. It is quite common in poor countries where people suffer from malnutrition (just as many CFers are malnourished prior to diagnosis.)