The lines are spoken by Viola near the end of Scene 5.
Here, Viola begs Olivia:
Lady, you are the cruel’st she aliveIf you will lead these graces to the graveAnd leave the world no copy.
Here, Viola is saying that Olive would be the cruelest woman alive if she would die before having children. The world would be worse off if there were no "copy" of Olivia left behind after her death.