In this excerpt from Macbeth, Lady Macduff tries to explain her husband's (Macduff's) sudden flight from their castle to their young son. Macduff has left his wife and children in the castle (with servants, presumably, but basically unprotected) so he can join Malcolm in England to plan the war to take the crown from Macbeth and restore it to Malcolm, the rightful heir. 

Here, we see Lady Macduff using an analogy, or more specifically, a metaphor, to compare Macduff's actions to those of a protective mother wren. She begins her comparison by saying that Macduff "wants the natural touch," meaning that what he has done is unnatural and that he has acted in a way that is different from the way parents act in nature or, more specifically, in the animal kingdom. She goes on to explain that even the weakest bird, here "the poor wren," will defend its young against even much stronger birds (the owl). Next, she returns to Macduff when she says "All is the fear and nothing is the love." Macduff's flight is in contrast to the actions of the wren. His actions,  according to his wife, are ruled by fear and not by love,  and the implication is that the wren will fight the owl because her love for her young is more important than her fear of the owl. She concludes by saying that Macduff's flight defies reason to compound her idea that it was also unnatural.