When you buy stock on margin, you put up cash for part of the purchase and borrow money from your broker for the rest. This practice can magnify your profit percentage when the stock price goes up, because you can control a far greater number of shares than you could with an all-cash purchase. But it can also magnify your losses when the price goes down. If it goes down far enough, you might get a "margin call" from your broker, in which case you’ll have to come up with more cash or sell all or some of your shares.