Margin call
A margin call is a notice from your broker: your account no longer has enough funds to hold the open positions. You are asked to add money or close some trades.
It is measured by the margin level: account equity divided by used margin, times 100%. Below 100% means a margin call. Next comes the stop out: at many brokers it sits near 50%, and positions are then closed automatically, usually starting with the most losing ones.
You have $5,000. You open 1 lot of USD/JPY with 3% margin: $3,000 is used and $2,000 is free. A $2,000 loss leaves $3,000 of equity against $3,000 of margin, the level hits 100%, and a margin call is very likely.
What not to do
Do not wait for the notice. Keep a cushion of free margin and cut the position early, while the level is far from 100%.
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