According to TheFreeDictionary, placenta previa is a condition in which the placenta is abnormally placed and either partially or fully covers the cervix. With a normal pregnancy, the placenta is attached rather high up on the uterine wall either on the front or back.
When a baby is ready to be born, the uterus starts contracting and the baby exits the womb via the vagina unless a C-section is needed. According to TheFreeDictionary, with placenta previa, the placenta covers the opening in which the baby would normally exit and causes problems with delivery. This condition almost always results in a C-section, or cesarean, delivery.