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A margin call means you have to send more money or close the position in order to raise your equity percentage. "Call" in this sense is more like you get a telephone call from your broker. When you buy on margin, you might be able to buy twice as many shares, but you absorb the loss on all of them. As the position goes against you, your equity percentage drops. As your equity percentage gets low, the firm's risk begins to increase. If you lose all of your money, it starts to be theirs on the line, so they make you put up more money or they close the position before you get past zero equity.
For example, if you have $50,000 at 2x margin, that will buy $100,000 worth of stock. If you bought 1000 shares of a $10 stock, you would reach zero equity when the stock dropped to $5/share. The margin call is designed to prevent this from happening.
Conventional wisdom says to never meet a margin call. Getting one is a big red flag that you messed up. It's better to close the position rather than send more good money into a bad investment.