Calpurnia says this quote. Spoiler alert for those who haven’t read the book (or for those who are reading it and haven’t gotten to this part of the book); stop reading here. This quote comes after Scout has offended a guest, the Cunningham boy. Calpurnia corrects her by saying this quote. It is meant to show Scout that being high and mighty in her own house around guests can offend them, and sometimes worse than the Cunningham boy is offended.
If you are mean to them in your house, how can they expect you to be any different outside of the home? This can also be taken to mean that home is where you are most likely to be yourself, so you should be careful how you act.