Miss Maudie teaches Scout a lot of things. For one, she teaches by example that you don't have to act like a debutante to be considered a fine, upstanding Southern lady. Thanks to Miss Maudie, Scout learns that it's okay for ladies to potter around the garden wearing a straw hat and men's coveralls.
Miss Maudie also teaches Scout that it's a sin to kill a mockingbird. What she means by this is that there are creatures who are put on this earth to do nothing but good. They do us no harm at all, and so it's sinful to do harm to them. Throughout the course of the book, we come across the human equivalent of mockingbirds in the shape of Tom Robinson and Boo Radley, who for different reasons are regarded with fear and suspicion by the people of Maycomb despite the fact that they're completely harmless.
Miss Maudie also teaches Scout the importance of deflating hypocrisy. One day, during tea with Aunt Alexandra's church group, a nasty piece of work by the name of Mrs. Merriweather utters a racial slur against her maid. This comes right after she praises the Reverend Everett for his missionary work among a poor, remote African tribe. So not only is Mrs. Merriweather a racist but a hypocrite to boot. To make matters worse, she makes a none too subtle attack on Atticus for stirring up African-Americans in town by defending Tom Robinson. Mrs. Merriweather's criticism is all the more galling as she's at Atticus's house, enjoying his hospitality. Thankfully, Miss Maudie is on hand to take Mrs. Merriweather down a peg or two:
His food doesn't stick going down, does it?