Final answer:
King Duncan names his son Malcolm the Prince of Cumberland, making him the heir to the Scottish throne, which poses a problem for Macbeth who has been told by witches that he will become king.
Step-by-step explanation:
King Duncan awards his son Malcolm the title of Prince of Cumberland. This act is significant because it signifies that Malcolm is now the heir to the throne of Scotland. For Macbeth, this presents a major obstacle to his ambition; the witches have prophesied that he will become king, but with Malcolm's new position, it seems that the path to the throne is obstructed. This fuels Macbeth's internal conflict and sets the stage for his subsequent actions to usurp the throne, actions stimulated by his desire for power and spurred on by Lady Macbeth's manipulation.