These lines appear at the end of the poem. At this point, the Virgin is "blest" and she has "laid her Babe to rest." Thus, Christ is sleeping and Mary is happy.
The poet then says that the "song should here have ending" -- meaning, the poem should end here, on this happy note.
He continues to describe the great star that announced the birth of Christ (the Star of Bethlehem -- the "youngest-teemed star" has now become "fixt." He is saying this star has become fixed in the sky, or has stopped moving. The star now continues to shine above the happy family, like a "handmaid lamp attending" the "sleeping Lord."
These lines are a fancy way to describe the happy Virgin, sleeping Christ, and shining star above them.
