When you buy stock on margin it means that you borrow money from your broker to purchase shares, and your investment is used as collateral. If you get a margin call from your broker, it usually means that the stock you purchased on margin has decreased in value. A margin call is basically a demand that you pay all or part of the loan with cash or by depositing other securities from an outside account with the broker. If you are unable to come up with the money, your broker can sell some of your assets to bring your account up to a specified minimum amount, or what is known as the minimum maintenance margin.

Buying stock on margin is risky, and you can lose money very quickly. For example, if you buy stock at $100 per share and the value drops to $50, you'll lose 50 percent of the money you invested. If you bought the stock on margin, you will lose 100 percent of your investment, and you'll also have to pay interest on the loan.

When you buy stock on margin, it is possible to lose more money than you invested because you must repay your broker the amount you borrowed, plus interest—even if you lose money on your investment. Brokerage firms also can sell the securities you bought on margin without notifying you. This can result in substantial losses—particularly if the broker sells the securities after the price has dropped, leaving you with no way to recoup the lost dollars once the stock price begins to rise again. You can read more about purchasing stock on margin and margin calls at www.sec.gov or www.finra.org.