What does this quote mean? "We were in front of the Radley place... High above us in the darkness a solitary mocker poured out his repertoire in...

In this quote Harper Lee is using the "mocker" (mockingbird) as a symbol of innocence.  The town has been through the trial and the children especially have been forced to grow up a little about what goes on in the real world.  They have had to face the fact that things are not always fair, and the town they live in has unfairness and evil dwelling there.  Scout, walking home hears the solitary mockingbird pouring out it's different songs in a "blissful unawareness."  Her eyes have been opened to what is going on in her town and how some people hate others simply because of the color of their skin, but the mockingbird is still innocent.  Tom Robinson is also innocent, but not in the eyes of the all male white jury.