Answer:
Duncan, the king, declares his intention to name Malcolm as the Prince of Cumberland and hence heir to king's throne.
Macbeth tells himself that now Malcolm stands between him and the throne.
Explanation:
In Act I Scene IV of Shakespeare's play "Macbeth", when Macbeth and Banquo return after winning the battle, Duncan, the king, announces that Malcolm will be his heir and Prince of Cumberland.
"We will establish our estate upon
Our eldest, Malcolm, whom we name hereafter
The Prince of Cumberland; which honour must
Not unaccompanied invest him only"
Macbeth apparently declares his joy, but when he is at some distance, he talks to himself that he either has to fall down by Malcolm or overleap him in order to become king himself.
"[Aside] 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"
