What's a simple way to say what infection from a retained fetus really is?

Uterine infection. The risk for infection does increase with an abnormal pregnancy. In the case of a retained fetus, the assumption is that the fetus is not alive and that there has been passage of at least some of the uterine contents - a partial miscarriage. In this case the likeihood is that eventually an infection would develope in the uterus - called chorionitis, amnionitis, or endomyometritis.
Endometritis. Inflammation of the lining of the uterus (womb) or its walls may occur when a fetus dies and does not come out. This requires intensive care by an obstetrician/gynecologist.