“Shoot all the blue jays you want, if you can hit ‘em, but remember it’s a sin to kill a mockingbird.” (Atticus Finch)
Mockingbirds are simply songbirds. They do not steal nests or eggs from other birds and simply exist and bring pleasure to anyone who hears their song. Atticus is using Miss Maudie's analogy to suggest how unjust it would be to accuse innocent men but guilty men "if you can hit' em" deserve everything they get. The main themes of To Kill A Mockingbird focus on injustices and prejudice.
The title does not have any literal significance but, as a symbol, killing a mockingbird is tantamount to destroying innocence. Tom Robinson, Boo Radley, Mr. Raymond, Jem and Dill have been effected through the evil they have encoutered.
Miss Maudie will refer to the fact that Mockingbirds "sing their hearts out for us" and killing them is a sin. Mr Underwood will compare Tom's death to “the senseless slaughter of songbirds.” Scout compares hurting Boo Radley to “shootin’ a mockingbird.” And not to be overlooked is the fact that Atticus and his family have the surname Finch. They may not be Mockingbirds but they are sensitive and subjected to the prejudiced environment in which they live.