What does Atticus mean when he says "she died free"?

He means that she died free of her morphine addiction.

"Weeks after the last day of reading, Atticus receives a phone call and goes to Mrs. Dubose's house for a long time. He comes back to announce that she is dead, and tells the children that she was a morphine addict. Jem and Scout's visits helped break her from her morphine addiction, which the doctors had prescribed for her as a painkiller for her illness. Atticus explained to his children that Mrs. Dubose is an example of true courage. Even though she knew she was going to die, Mrs. Dubose wanted to be free of her addiction."