Mary was pledged to be married to Joseph the carpenter.  Before they were married, it became known that Mary was with child, so Joseph planned to cancel the marriage.  But before he did, an angel visited Joseph in a dream and told him not to be afraid to take Mary as a wife, because the child in her womb was conceived by the Holy Spirit.  The angel told Joseph that Mary would have a son, and that they were to name him Jesus, because he would save his people from their sins.  Joseph did as the angel commanded.