Margin is borrowing money from your broker to buy a stock and using your investment as collateral. Investors generally use margin to increase their purchasing power so that they can own more stock without fully paying for it.But margin exposes investors to the potential for higher losses.ut if you bought the stock on margin  paying $25 in cash and borrowing $25 from your broker  you'll earn a 100 percent return on the money you invested. Of course, you'll still owe your firm $25 plus interest.