Although Jem and Scout won't understand the real reason for Mrs. Dubose's change until Atticus explains it to them after her death, it is because she has successfully rid herself of her longstanding morphine addiction. Her drooling has ceased and

... Although her fits had passed off, she was in every other way her old self.  (Chapter 11)

Instead of sleeping and being only half-awake, she "would become bored and pick on us." She had returned to the mean old lady who regularly accosted Jem and Scout from her porch. After her death, Atticus explains to Jem the reason for her misery and for her need to have someone read to her: It was because of her going cold turkey from the drug, and she found it easier for the reading to be used as a distraction. Atticus told Jem that Mrs. Dubose "was the bravest person I ever knew." She could have died without pain had she chosen to continue taking Dr. Reynold's morphine, but she decided that she would die "beholden to nothing and nobody." Her courage was not like that of "a man with a gun in his hand"; instead, it was a special type of moral courage.

"It's when you know you're licked before you begin but you begin anyway and you see it through no matter what. You rarely win, but sometimes you do. Mrs. Dubose won..."  (Chapter 11)