What is a margin call ?

A broker’s demand on an investor using margin to deposit additional money or securities so that the margin account in brought up to the minimum maintenance margin. This is sometimes called ‘fed call’ or ‘ maintenance call. You would receive a ‘MARGIN CALL’ from a broker if one or more of the securities you had bought (with borrowed money) decreased in value past a certain point.
Example
Let’s say you open a regular Forex account with $3,000 (not a smart idea). You open 1 lot of the EUR/USD, with a margin requirement of $1000. Usable Margin is the money available to open new positions or sustain trading losses. Since you started with $3,000, your usable margin is $3,000. But when you opened 1 lot, which requires a margin requirement of $1,000, your usable margin is now $2,000