Margin call
A margin call is your broker's alert when your margin level drops below a set threshold: your capital barely covers your positions any more.
Historically, a margin call was a phone call asking you to deposit funds. Today it is an automatic alert on the platform when your margin level falls below a threshold, often 100%.
At that point you have three options: deposit money, close some positions or let it run. The third is the most dangerous: if the market keeps going against you, the stop out will close your positions for you.
A margin call is the symptom of excessive exposure or a position without a stop. With sound risk management, it should never happen.