Just as the King is commenting on the treachery of the former Thane of Cawdor, in comes the new Thane of Cawdor, Macbeth. The King greets Macbeth as "worthiest cousin!" (1.4.14) and says in several different ways that he can't thank him enough. Macbeth answers with heroic modesty that "The service and the loyalty I owe, / In doing it, pays itself" (1.4.22-23). That is, it's payment enough to know that he did the right thing as a loyal servant of the King. Then Macbeth adds, Your highness' part
Is to receive our duties; and our duties
Are to your throne and state [stateliness, dignity] children and servants, Which do but what they should, by doing every thing
Safe toward your love and honour. (1.4.23-27)
"Safe toward" means "to secure" or "to safeguard"; the idea is that it is every subject's duty to do everything he can for the king, both to keep the king safe and to earn the king's love and respect. Macbeth's speech pictures King Duncan as the loving father of a happy family, but Macbeth is already thinking about killing him.