In chapter 5 of "To Kill a Mockingbird", Dill and Jem start hanging out together, which leaves Scout left out of the action. So, Scout spends a lot of time sitting with Miss Maudie on her front porch, chit-chatting about various subjects. Since Boo Radley is a fascination with all of the kids, Scout naturally brings it up to Miss Maudie, to see if she has any new information to add to the sujbect. Miss Maude tells Scout that old Mr. Radley, Boo's father, was a "foot-washing Baptist." She goes on to explain that "footwashers believe anything that's a pleasure is a sin." So, it is a religious sect that strictly follows the Bible, and keeps any form of pleasure out of their lives. They are so strict about it that Miss Maudie tells Scout that one day some foot-washing Baptists walked by her house and told her that her "flowers were going to hell" right with her; I guess planting beautiful flowers, and admiring something so lovely is a pleasure, and hence, a sin. The foot-washers also "think women are a sin by definition," probably because Eve ate the apple first in the garden of Eden, and tempted Adam to eat of it too, causing mankind's fallen state. So, Scout gets a bit of background to Boo's very strict upbringing, which makes his father's extreme reaction to his pranks about town that much more understandable. I hope that helps!
In Chapter 5, Scout asks Miss Maudie why Boo Radley never comes out of the house. Miss Maudie proceeds to tell Scout that Boo's father was a "foot-washing Baptist." Miss Maudie says that "Foot-washers believe anything that's pleasure is a sin" (Lee 28). They are essentially religious fanatics who have a strict interpretation of the Bible. Foot-washers continually ridicule Miss Maudie for working in her garden because they believe that she should be spending the majority of her time indoors reading the Bible. Upon hearing this, Scout tells Maudie that she's the best lady she knows. Maudie thanks Scout and proceeds to tell her that "foot-washers think women are a sin by definition" (Lee 28). Maudie tries her best to explain to Scout the behavior of people who take the Bible literally. Scout is too young to understand how religious fanatics are often intolerant of others who do not share the same beliefs and can come across as callous individuals.