In stock futures trading, a margin call takes place when a trader has been using a margin account to purchase stocks, and when the account has reached the point where it must be paid back.
Having a margin account means that the person trading is essentially borrowing funds from the broker to purchase stocks. How margin accounts work varies depending on the broker and the contract with the specific trader, but in general they work separately from regular cash accounts. A good margin account will often let the trader borrow up to 50% of his account value (or more) towards the price of a stock. This means if you had $100,000 in your account, you could potentially have up to $50,000 in "margin" and could invest that additional $50,000 as if it were your own, although with margin interest payable to the broker.
If, however, the borrowed amount grows too large, either because the trader is failing to pay back into the account or because the prices of the stocks have changed adversely, a margin call may be brought into play. This is when the brokerage essentially alerts the trader that he or she owes too much on the account.
Paying a margin call means that you add dollars to the account, in effect paying the broker back in cash what you have borrowed. If you are unable to do this, you will be forced to sell off some shares in order to replace some of the borrowed funds.
Typically, when a margin call is issued, you must respond immediately. If you fail to meet a margin call within a reasonable period of time, which could be as little as one hour, your brokerage firm may close out your positions to reduce your margin deficiency. If your position is liquidated at a loss, you would continue to be liable for that loss. For this reason, you stand to lose a substantial amount of money if you don't respond in a timely manner to a margin call.