What does Macbeth mean when he says, "There's not a one of them but in his house/ I keep a servant fee'd" (3.4.132-33)?in Macbeth
This quote takes place in Act Three, Scene 4, during a conversation between Macbeth and his wife regarding Macduff's loyalty. Macbeth initially asks his wife what she thinks about Macduff disobeying his orders—even though he has yet to give him a command—and tells Lady Macbeth that he has indirectly heard of Macduff's enmity. Macbeth proceeds to tell his wife,
"There’s not a one of them but in his house I keep a servant fee’d" (Shakespeare, 3.4.137-38).
Macbeth is essentially telling his wife that he has paid servants, who are strategically located in each of the Scottish lords' homes, to spy for him. The fact that Macbeth is paying people to spy on the Scottish lords emphasizes his anxiety and fear that he will lose his throne. Macbeth is well aware of the witches' prophecies, and he attempts to prevent anyone else from usurping power, which is why he pays servants to spy for him. After telling his wife that he is paying spies to work for him, Macbeth mentions that his safety is his utmost concern.
What this is telling us is that Macbeth does not really trust anyone. He is telling his wife that he has paid spies in the homes of every lord in Scotland, presumably.
This comes up because Macbeth says that Macduff will not come when Macbeth orders him to come. He has not actually ordered Macduff to come and so Lady Macbeth wonders why he knows that Macduff will not obey. That is the point when Macbeth speaks the line you mention.
When he says "not a one of them" he is referring to the lords. By saying "a servant fee'd" he is referring to someone he has paid (given a fee) to work for him as a spy.