This refers to the virgin birth of Christ. For information on the biological phenomenon, please see parthenogenesis.
The doctrine asserts that Jesus Christ was conceived in the womb of his mother, the Virgin Mary, without the participation of a human father. Instead, the conception took place miraculously when the Holy Spirit "overshadowed" Mary. This was not understood to mean that the human body of Christ was created ex nihilo (from nothing), for the tradition of the Church is that Christ "took his flesh from Mary." This is also understood to be a miracle, something not possible without divine intervention.
This doctrine is frequently confused with the doctrine of the Immaculate Conception. The latter, held by Roman Catholics, states that the Virgin Mary was conceived "without the stain of original sin". Mary, however, unlike Christ, was conceived in the ordinary way: i.e. she had a human father as well as a human mother (whose names, according to Catholic and Orthodox tradition, were 'Joachim' and 'Anna').
Another reason that Christians consider the virgin birth to be significant is that it shows Jesus' divine and human natures at once united, paving the way for all of humanity to be united with God. Eastern Orthodox tradition says that from the time Jesus was born, the flaming sword was removed from the Garden of Eden, making it possible for humanity to re-enter Paradise.