The Virgin Birth refers to the virgin birth of Jesus as recorded in the Gospels of Matthew and Luke (Matt 1:18 - 1:24, Luke 1:26 - 1:38).
The aforementioned Gospels state that Jesus was not conceived through the normal biological process, but was conceived by the Holy Spirit touching Mary to create a baby forming in her womb. She was a virgin, having never had intercourse with a man. In this way Jesus was born without the original sin of Adam being passed down from generation to generation. Matthew tells us this fulfills the prophecy the Lord made to Isaiah in the Old Testament in Isaiah 7:14
"Behold a virgin will be with child and will give birth to a son"
Matthew used the Greek Septuagint, the Greek translation of the Old Testament common in Palestine at that time.
The Unification Church does not accept the traditional Christian doctrine of the virgin birth, teaching rather that Zechariah was the father of Jesus.  Unification Theology likens this to the story of Tamar who became pregnant by her father-in-law Judah.