A margin call in forex trading is when a broker requests the trader to deposit additional funds into their trading account to meet the minimum margin requirement. If the trader fails to do so, the broker may close out some or all of their positions. This is done to mitigate the risk of the trader defaulting on their trades, and to ensure that the broker is able to cover any losses incurred on the trader's behalf. Margin calls can occur when the value of the trader's positions declines, and their account no longer has enough equity to support their open trades.