here is a great video that really helped me understand the cases of German. https://www.youtube.com/watch?v=8VowGvnRU_s&list=PLYzp2xhTw9W1Xfjp8_0oxsULeUw5g80TH&index=1
Because you are saying to someone else they are bringing the apple. If you were going to bring the apple, you would say in English, "I'm bringing the apple" whereas, as you are telling someone else to bring an apple, you say "You are bringing the apple". Hopefully this may help.