Answer:
It shows how he is growing stronger in the face of murder. The dagger points him towards duncans room as it floats in front of him. He is so caught up in the adrenaline of the pre-act of murder,that his mind is almost pushing him to do it. He sees it, but cannot grasp it. he believes that to grab the dagger, and to be a real man, he must go through with killing one of his kinsmen.