In act 4, scene 3, Malcolm, the rightful heir to Scotland, returned from England with an army and fighting to wrest the throne from Macbeth, wants to ensure that Macduff's loyalties are first and foremost to Scotland, not him.
He does this by pretending to be a morally terrible person, greedy, lustful, and violent, out to use the country and all its resources for his own pleasure and personal profit, with no concern about the fate of anyone but himself and those few personally loyal to him.
Macduff, who desperately wants to defeat Macbeth, the man who cold-bloodedly had his wife and children slaughtered, tries to hang on to his loyalty to Malcolm, who has the best shot at displacing his hated enemy. However, eventually, his own sense of right and wrong prevails and Macduff recoils from Malcolm, saying he could never support such a man as Malcolm describes himself to be. Macduff puts the well-being of Scotland ahead of personal loyalty to any man or any personal revenge agenda.
Reassured of Macduff's moral fiber, Malcolm explains to him that he is not at all like the person he just described. Malcolm calls himself a "meek" (kind and compassionate) and just ruler. He simply had to know that Macduff cared about Scotland above all.
Through this scene, Shakespeare sharply differentiates Malcolm from Macbeth, who only cares about his own power and is ready to throw Scotland under the bus to hang on to it. Macbeth is an evil tyrant; Malcolm shows he will, like his father, be an exemplary ruler.