What does Miss Maudie mean when she says it's a sin to kill a mockingbird?

Miss Maudie says: "Mockingbirds don’t do one thing but make music for us to enjoy . . . but sing their hearts out for us. That’s why it’s a sin to kill a mockingbird."

So, you shouldn't hurt something or someone that only tries to be helpful.

Miss Maudie is trying to help Scout understand that you don't hurt the innocent, you protect them. Mockingbirds are innocent birds unlike other birds who are harmful (especially to farmers) so while other birds can take care of themselves, Mockingbirds need our care. This is foreshadowing the end of the novel and Tate's treatment of Boo. Tate tries to protect Boo because all he did was try to help.