"Go short" means you sell first, then buy it back later. If you think AAPL is going lower, you can borrow shares from your broker and sell those. And when it goes down, you buy back shares at lower price and return them to broker with interest.
To simplify - when you long, you bet on price going up. When you short, you bet on price going down. If you went short and price dropped 5%, your profit is 5% (minus commission, slippage, and some other stuff).