A margin call is when your broker gives you a nasty telephone call when you least expect it and asks (sometimes politely) if you would like to put up some more money to cover your margin loan or he will sell your stocks for you the same day.
Well, maybe "when you least expect it" is not the best of terms, but it's a euphemism for how a lot of margined out investors will be caught on the day of reckoning in the stock market. The right way to explain it is that if the price of your shares fall, the value of your "margin" drops and you must top it up within a certain time limit or your broker sells your shares for you and then still tells you that you still owe them some money.
One old saying in the stock market amongst investors who have been around long enough to know better is "never meet a margin call"