This quote obviously refers to the title, To Kill a Mockingbird. Scout notes that this is the one time that she can recall Atticus saying it was a sin to do something. Miss Maudie clarifies the point.
Mockingbirds don’t do one thing but make music for us to enjoy. They don’t eat up people’s gardens, don’t nest in corncribs, they don’t do one thing but sing their hearts out for us. That’s why it’s a sin to kill a mockingbird. (Chapter 10)
The best examples of mockingbirds in this novel are Boo Radley and Tom Robinson. Both men only try to help people, expecting nothing in return. The message is that it would be a sin to kill (or harm) such people. One could therefore conclude that the jury (and the town) sinned by convicting Tom. However, at the end of the novel, there is some redemption, particularly at the urging of Heck Tate, when Boo Radley is spared unnecessary attention and possible scandal. In Chapter 30, Heck says, "To my way of thinkin’, Mr. Finch, taking the one man who’s done you and this town a great service an‘ draggin’ him with his shy ways into the limelight—to me, that’s a sin."
In other words, it is a sin to harm those who do nothing wrong, help when they can, and/or keep to themselves. Additionally, Boo and Tom are, like the mockingbird, rather defenseless. It would be an even greater sin to harm innocent people who, for one reason or another, have little means to defend themselves.