What does the title mean in 'To Kill A Mockingbird'?

The Mockingbird symbolises innocence, thus its death ('To Kill A Mockingbird') signifies the death of the children's innocence as they are exposed to the racial prejudices and the consequences of those prejudices (Tom Robinson's death) in Maycomb. The notion of the mockingbird being killed reflects the novels genre as a bildungsroman novel, or coming-of-age, as the Finch children progress from childhood to maturity, leaving their innocence behind.