In chapter 24, Aunt Alexandra hosts her missionary circle and invites Scout to participate in the social event. Scout, who has never attended a function similar to the missionary circle, is quite nervous and wears a dress, which is extremely rare for a tomboy like herself. When Miss Maudie attempts to make Scout comfortable, she asks,

You’re mighty dressed up, Miss Jean Louise...Where are your britches today? (Lee, 233).

Scout regrettably responds, "Under my dress" (Lee, 233). Scout mentions to the reader that she had not meant to be funny, but the other ladies laughed. Unlike the other ladies, Miss Maudie looks "gravely" down at Scout because she realizes that Scout was not making a joke. Miss Maudie's reaction is significant because it illustrates her relationship with Scout. Miss Maudie treats Scout like an adult and truly understands whether or not Scout is being serious or joking. In this case, Scout had not meant to make a joke, and Maudie's solemn reaction makes Scout aware that she had made a mistake. Maudie makes it clear to Scout that her answer was wrong and some information is best left unsaid. In contrast, the other ladies do not have a close relationship with Scout and assume she is attempting to make a joke. 

In the beginning of chapter 24, Scout is invited to be a part of Aunt Alexandra's "missionary circle" where the women gather to gossip and have tea and cookies.  For this occasion, Scout has dressed up in a dress and looks nice.  Miss Maudie is the one who says that she looks nice.  She asks her where her britches are--which is referring to her overalls that she always wears.

"'Under my dress.'"  I hadn't meant to be funny, but the ladies laughed.  My cheeks grew how as I realized my mistake, but Miss Maudie looked gravely down at me.  She never laughed at me unless I meant to be funny."

When she said she had it under her dress, she was referring to her underwear, not her overalls.  She misunderstood.  However, the women all laughed at her--but not Miss Maudie.  She knew Scout was trying to behave and wasn't trying to be funny.  Maudie is a true friend.