What announcement does the King make to everyone present and what is Macbeth's reaction? (Act 1, scene iv)

In the play Macbeth, by William Shakespeare, Duncan says
several important things in Act I, scene iv, but one is extremely important to the
kingdom—and Macbeth, especially.

First, when Macbeth and
Banquo arrive, Duncan says that he is indebted to Macbeth and will continue to reward
him for all the soldier has done for him. Duncan also extends his thanks to
Banquo.

However, Duncan's big
announcement (that will have an enormous impact on Macbeth's hopes of being
king one day) is the name of Duncan's successor. The King announces that his son,
Malcolm, will be his heir to the throne, and he bestows the title of Prince of
Cumberland upon him.

Whereas Macbeth would only have had to
deal with Duncan standing between him and the throne, now he realizes that Malcolm
creates another obstacle.

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The Prince of Cumberland! That is a step / On
wich I must fall down, or else o'erleap, / For in my way it
lies.

Malcolm is blocking
Macbeth's way. His designation as heir to the throne will either thwart Macbeth's plans
to become King, or he will have to climb over Malcolm in order to get what he
wants.