If the mother is RH negative, and the father is RH positive, then there's a possibility that the baby is RH positive. If this is the case, during birth if the mother and babies bloods mix, the mother's blood will attack the RH positive proteins because it is a foreign substance to them.
It can harm the first child, but the real problems come with other children. If the body has recognized the RH positive proteins in the first birth, it will seek them out. If the second child has RH positive blood, the mothers blood could severely harm the second baby by constantly attacking the positive proteins, and decreasing the blood count.
Hopefully this makes sense....its kind of a confusing process.