it is really self-explanitory, the fish (usually cichlids) get a hole in their head, is a intestinal tract infection.
It is caused by bad water quality, and is difficult to treat and cure, Snowcichlid has had sucsess, but in my experience it slowly kills the fish over a period of months.
It also can cause a lack of appitite, a loss of color, and the fish to look like a swimming skeliton
Otherwise known as HEXAMITA here is a couple of helpful links to help you find out more. http://www.fishyfarmacy.com/previous%20articles/hexamita.html