Act 4, scene 2 of Macbeth takes place in Fife, where Macduff's family has been left behind while he has gone to fight alongside Malcolm and the English army to challenge Macbeth for the Scottish crown. In this scene, Lady Macduff speaks to one of her children, a young son, about Macduff's flight. By the end of the scene, murderers ordered by Macbeth have entered the castle and have killed Macduff's entire household.
When the scene opens, Lady Macduff is speaking to Ross. While he tells her Macduff must have honorable motives for leaving his family untended, the lady instead feels Macduff is not doing his duty to his family. She states,
Wisdom! to leave his wife, to leave his babes,
His mansion and his titles in a place
From whence himself does fly? He loves us not;
He wants the natural touch: for the poor wren,
The most diminutive of birds, will fight,
Her young ones in her nest, against the owl.
She counters Ross's claim that Macduff is wise and describes what her husband had done--leave his family to fend for themselves in what has become a dangerous country. She interprets his behavior to mean "He loves us not" because if he loved them, their protection would be his top priority. She incorporates an analogy, suggesting that even a small, relatively weak bird will not leave her family alone to be killed by "the owl." This analogy implies that what Macduff has done is unnatural ("wants the natural touch").
Lady Macduff proceeds to tell her son that Macduff is dead (he is not), because his abandonment of his family, in her eyes, signals his betrayal. The figurative language associated with birds continues, as she refers to her son as "poor bird." Lady Macduff wonders what her son will do without a father, and he replies, continuing the bird analogy:
Son: As birds do, mother.
LADY MACDUFF: What, with worms and flies?
Son: With what I get, I mean; and so do they.
The son claims he will survive without a father; he will make do. While his mother's question is a negative take on living "As birds do," the son's response highlights the resourcefulness of birds, and by extension, of himself and his family in the way they can move on without Macduff's presence. The son asks whether Macduff is a traitor and Lady Macduff responds that he is because he has betrayed his king (albeit a tyrannical and unjust one). Lady Macduff insists that she could find another husband easily enough, but it would be more difficult for her son to find another father. It seems that Lady Macduff knows the centrality of family, and of the role of the father in his children's upbringing, but she also feels that her husband should have been loyal to his country and definitely should not have left his family behind to bring a rebellion against Macbeth.
Ultimately, this scene is tragic because Macduff's whole family is killed, and when is told of this in a later scene in act 4, he is devastated but also newly motivated to fight Macbeth. Macduff has to reconcile with himself to the guilt he feels at having left in such a hurry that his family was not secure.