"The prince of Cumberland! That is a step
On which I must fall down, or else o'erleap,
For in my way it lies. Stars, hide your fires;
Let not light see my black and deep desires.
The eye wink at the hand, yet let that be
Which the eye fears, when it is done, to see."
The foreshadowing referenced within the aside is that Macbeth is talking about stepping over Malcolm or giving up. It's foreshadowing because Macbeth kill's Malcolm's father, Duncan.