In chapter 16, Jem, Dill and Scout are watching people pass in front of their house on the way to the courthouse. A "wagonload of ladies" goes by in plain dress, wearing bonnets and long sleeves. Jem confidently explains that these are Mennonites, that they "don't have buttons," that they all have blue eyes and the men are forbidden to shave after marriage.
This scene comes after the children have faced down the "mob" and are in a kind of heightened state. Jem's knowing explanation is perhaps a reflection of this new confidence and sense of maturity, particularly his somewhat naughty observation that the Mennonite women like to be tickled by their husband's beards. It is also hilariously naive, a quality that stands in contrast the the serious nature of the trial these people have come to town to witness.