You receive a margin call in forex when your available capital drops below margin requirements. This usually occurs when the market has moved against your open positions and depleted your deposited capital. When you receive a margin call, the broker closes your open positions to prevent your equity from dropping to a negative value and requests that you deposit more funds if you want to continue trading.
Some brokers offer tiered margin warnings. That is, when your positions are in loss and you are nearly reaching half of your margin requirements, the broker sends you a margin warning, not a margin call. This gives you the chance to either close the positions early or deposit more funds. With a margin warning, the broker does not close...