One of three windows in a row dedicated to Mary the Mother of God. The story as told in Luke can be found below the picture.
From Luke 2 the story:
1 And it came to pass that in those days there went out a decree from Caesar Augustus that the whole world should be enrolled. 2 This enrolling was first made by Cyrinus, the governor of Syria. 3 And all went to be enrolled, every one into his own city. 4 And Joseph also went up from Galilee, out of the city of Nazareth, into Judea, to the city of David, which is called Bethlehem: because he was of the house and family of David. 5 To be enrolled with Mary his espoused wife, who was with child.
6 And it came to pass that when they were there, her days were accomplished that she should be delivered. 7 And she brought forth her first born son and wrapped him up in swaddling clothes and laid him in a manger: because there was no room for them in the inn.
8 And there were in the same country shepherds watching and keeping the night watches over their flock. 9 And behold an angel of the Lord stood by them and the brightness of God shone round about them: and they feared with a great fear. 10 And the angel said to them: Fear not; for, behold, I bring you good tidings of great joy that shall be to all the people: 11 For, this day is born to you a Saviour, who is Christ the Lord, in the city of David. 12 And this shall be a sign unto you. You shall find the infant wrapped in swaddling clothes and laid in a manger.13 And suddenly there was with the angel a multitude of the heavenly army, praising God and saying: 14 "Glory to God in the highest: and on earth peace to men of good will."
The rest of the story from Luke 2 is found on the New Advent site. Link to the site.