A symbol is something concrete that stands for a more abstract characteristic or concept. In this scene, one of the most obvious symbol is that of birds. Birds symbolize different things depending on the type of bird with birds of prey generally representing evil or danger while smaller birds represent freedom and innocence.
Lady Macduff uses a bird to describe her feelings about her husband and his quick departure. She notes that
The most diminutive of birds, will fight Her young ones in her nest, against the owl.
She intends to make the audience feel as if Macduff has shirked his duty, leaving his young ones in danger. He has no courage, according to his wife, and no drive to save his family.
The effect of this symbol is to further the idea of flight in the play. Malcolm and Donalbain have "flown" to England and Ireland. Macduff has now "flown" as well. Lady Macduff is warned to flee but choses not to, asking "whither should I fly?" She does not understand that she is in danger, thinking instead only of her anger at her husband.