In the novel To Kill a Mockingbird, mockingbirds are symbolic of innocence and goodness. They’re often described as singing outside of windows, but they’re unable to see inside of the houses they sing outside of.
Mockingbirds are also a symbol of how people see racism in the novel—they only see what’s on the surface, not what’s underneath. So if you’re mocking someone for something that isn’t their fault, it’s like you’re mocking them for being black or poor or whatever your prejudice is against them.