Annaliza is correct. The expression does mean stop harassing or bothering someone. But, the expression is figurative (paints a mental picture), not literal (exactly what the words say). To understand the figurative expression, imagine the literal expression "get off my back" which literally would mean that someone jumped on your back and created a heavy load (a great burden) for you to carry. So, if someone were harassing you or bothering/annoying/pestering you, that would be stressful for you, and you would want to tell that person to stop = get off my back.