An episiotomy is a small incision in the tissue between the vagina and the anus just prior to delivering the baby's head. The purpose of the incision is to enlarge the birth opening during the pushing stage of labor. It's purpose may be to keep the perineum from tearing or to hasten the delivery.
Many doctors do routine episiotomies on every patient. Others evaluate the need before making the making the incision. Some doctors will massage and stretch the perineum to avoid the need for an episiotomy.
You can decrease your chances of an episiotomy by stretching the tissue in the weeks prior to birth by doing squats, sitting cross legged, toning the kegel muscle, and massaging your perineum with olive oil or grapeseed oil.