Scout hears Mrs. Merriweather say the following:
“Oh that. Well, I always say forgive and forget, forgive and forget. Thing that church ought to do is help her lead a Christian life for those children from here on out. Some of the men ought to go out there and tell that preacher to encourage her.”
She asks if they are talking about Mayella and is told that they are talking about Helen Robinson, Tom’s wife. Scout assumes they are talking about Mayella because it is actually Mayella who has done something wrong. Mayella tried to have an inappropriate relationship with Tom. She is the one who took advantage of Tom’s generosity and friendship. Mayella was also solely responsible for all of her children.
Since Helen Robinson did nothing wrong, it is hard for Scout to see what she would be forgiven for, since Scout does not realize how deeply racial prejudice runs. It does not make sense to Scout that anyone would blame Helen when Mayella is the one who has done something wrong. This exchange is part of Scout’s education about prejudice and how unfair it is.
In Chapter 24 of Harper Lee's To Kill a Mockingbird, Aunt Alexandra entertains a meeting of the local ladies' missionary society. This group includes Mrs. Grace Merriweather, Miss Rachel, Miss Stephanie Crawford, and Miss Maudie. Despite the fact that the Mrs. Merriweather speaks extensively on the horrors of life among the Mrunas, a woman ("Gertrude") sitting near her manages to turn the conversation to local concerns with a single comment.
When Scout overhears Mrs. Merriweather mention "sin and squalor" and "forgive and forget," as well as Merriweather's comment that what
...the church ought to do is help her lead a Christian life for those children from here on out. Some of the men ought to go out there and tell that preacher to encourage her...,
Scout assumes that Mrs. Merriweather is referring to Mayella Ewell. The notion that Merriweather might be referring to Helen Robinson never occurs to Scout. The idea of judging Helen based on unfair and unjust racial stereotypes is not a concept Scout can understand. She knows that Mayella is the the one lying, not Helen, and that the Robinson family is kind, clean, and churchgoing; Scout is also aware that the Ewell family is often cruel or otherwise "bad," lives in filth and chaos, and exhibits no religious or moral tendencies.