Scout's text message to Jem would be a confrontation about what Jem thinks of Scout as he grows up. Jem adapts the idea that Scout is just a "girl", giving her a label that states that women are of a lesser value than men. This is very common during that time era to treat women below the superior status of men.
From the very beginning of the story, Jem got into less trouble than Scout did. In other words, he was very obedient and always listened to his father, Atticus Finch. This message is a command to leave the Radley place alone. Atticus wanted this to be a lesson to respect the privacy of not just the Radley Family, but anyone else for that matter.