Understanding the question becomes easier when you put it in context.
Aunt Polly has been calling and looking for Tom without any response. She is becoming rather exasperated with the search, so calls his name more loudly. This time, she hears a soft rustle and turns around just in time to catch Tom as he was trying to sneak behind her and make his escape. He had been hiding in a closet.
Aunt Polly asks what he had been doing in the closet and Tom claims he wasn't doing anything. Aunt Polly doesn't believe him, and there is evidence to give her cause to doubt his innocence.
'Nothing! Look at your hands. And look at your mouth. What is that truck?' 'I don't know, aunt.' 'Well, I know. It's jam-that's what it is. Forty times I've said if you didn't let that jam alone I'd skin you.
"Truck" in this use is something unidentified but obviously there. Tom had been eating forbidden jam. Aunt Polly hadn't yet realized what it was when she asked the question, but she could tell he had something in his mouth that wasn't supposed to be there!