A margin call is also known as MC, it is a warning message that occurs when a trader’s account is running out of sufficient funds to sustain their current floating / active positions. If the market moves against a trader’s position(s), additional funds will be requested through a "margin call". If there are insufficient available funds, the trader's open positions will be closed out
If a trader’s Equity (Balance - Open Profit/Loss) is below certain margin level which is the amount required to support open positions, then the trader’s positions will automatically be closed.
This is calculated as follows for the MetaTrader 4 platform: Equity / Margin = < 100%
It is advisable to all trader to maintain sufficient margin in their trading account should they need to maintain multiple trading positions over a long period of time.