The idea that Mary was a virgin when her son Jesus was conceived and born, making his existence a miracle caused by God. (Note: this does not mean she was always a virgin; she married Joseph and had children with him, something many Christians forget even though it's in the Bible that Joseph and Mary had several children.) Not the same as the doctrine of Immaculate Conception, which has to do with Mary's parents and was what made her a person pure enough to bear the Son of God.