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.
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.