It is informal speech. To give something back means to return it after receiving it. Here "give-back" is a verb phrase converted to a noun by putting a hyphen between the words. It means "the thing which is to be returned".
Here the accused have been given into the hands of the jurors who will decide their fate. Most of them they will hand over to punishment, presumably prison. But a few they will "give back" in the sense that they will let them go free. These are the "give-backs".
Abbie is not simply saying that Lee is likely to be acquitted. She has invented a term for what she sees as his role in these judicial proceedings.