A margin call is a term for when a broker requests an increase in maintenance margin from a trader, in order to keep a leveraged trade open.
When trading using a margin account, a broker may at any time revise the value of the collateral securities margin. For more info on margin trading click here. This can be revised for several reasons, primarily market conditions. If this results in the market value of the collateral securities for a margin account falling below the revised margin, the broker or exchange immediately issues a "margin call", requiring the investor to bring the margin account back into line. To do so, the investor must either pay funds known as the call into the margin account, provide additional collateral or liquidate some of their positions. If the investor fails to bring the account back into line, the broker will forcibly take action to keep you within the margin.
If a margin call occurs unexpectedly, it can cause a domino effect of selling which will lead to other margin calls and so forth. This situation most frequently happens as a result of an adverse change in the market value of the leveraged asset or contract. It could also happen when the margin requirement is raised, either due to increased volatility or due to legislation. In extreme cases, certain securities may cease to qualify for margin trading; in such a case, the brokerage will require the trader to either fully fund their position, or to liquidate it.