Setting up a margin account at a broker/dealer allows you to buy on margin. Not something that we recommend unless you are a very experienced investor and comfortable living with a much higher level of risk. You are basically ‘borrowing’ from the broker in order to buy more shares of a stock. When the broker has a “margin call” it means you must pay the money you borrowed. Margin calls can occur at any time but especially when the market has turned down. This is likely to be when the investor has lost money and must liquidate something in order to pay the margin. You can see why this has a high level of risk.