The title is an example of a metaphor.
Titles often contain metaphors. A metaphor is an indirect comparison. In this case, we are not literally talking about shooting mockingbirds.
The title comes from when Scout and Jem get guns for Christmas and Atticus tells them for the first time not to do something because it’s a sin.
Shoot all the bluejays you want, if you can hit 'em, but remember it's a sin to kill a mockingbird. (ch 10)
Miss Maudie explains that mockingbirds do not do anything to hurt anyone. They just make beautiful music.
Mockingbirds are innocent creatures sometimes harassed by others. This brings us to the symbolic nature of the title. The characters that are mockingbirds are Boo Radley and Tom Robinson. Neither of them are dangerous and they both are helpful, but they are both victims of society.