What does atticus say about mocking birds and what does he mean?

In "To Kill a Mockingbird," Atticus Finch explains that it's a sin to kill a mockingbird because they do nothing but sing and bring joy, symbolizing innocence and goodness. He uses this metaphor to convey that harming innocent people, like Tom Robinson or Boo Radley, is morally wrong. Essentially, Atticus teaches his children to protect those who are vulnerable and to stand against injustice.