short is where you take the opposing position in which you buy high sell low. you place your order to short a stock at $50. if the stock plummets down to $25 and you want to take profit, the term used would be buy to cover; you buy back your position at a lower cost and you make a profit within the difference between them.
pretty much when you first open a short position you do not technically "own" the stocks, you are borrowing them from your broker and selling them to optimistic buyers on the way down. once the price per share is below where you borrowed, you come back and cover your position at a lower cost making money.
short a 100 shares of stock XYZ at $50
price of XYZ per share drops to $25
you've made that difference in selling (50-25=25) thus yielding you a $25 per share return times your position (25x100=2500)