A hillbilly is a particularly unworldly country person. If you grew up in a remote rural area, visiting a big city for the first time can make you feel a bit like a hillbilly.
It's not nice to call someone a hillbilly — it's an insult, implying stupidity or at least a profound lack of sophistication. You might picture a hillbilly wearing overalls and chewing tobacco, uneducated and inexperienced. Today the word includes anyone who seems to fit this stereotype, but originally it meant specifically "southern Appalachian U.S. resident," from the hilly terrain in that area and the first name Billy.