Prolapse usually happens when a hen starts laying too early, lays unusually large eggs, or is excessively fat. It can also come from a hen being egg bound.
Part of the hens oviduct is forced out of the vent, and is visibly exposed.
If caught early enough, you can isolate the hen, and apply a hemorrhoid cream to the affected area until the condition improves. If the hen is left in the pens with the other birds, you will have to deal with the other birds picking at the exposed vent. They can actually start pulling out her oviduct and intestines, which can lead to fatality.
This can be caused by deficiency of Calcium or Phosphorus.