Margined FX trading is where a deposit of cash or other acceptable security is required to secure the performance of simultaneous buying of one currency and selling of another. The face-value of the position is multiplied by the margin factor.
Currencies are quoted in pairs (e.g. Euro versus the US dollar), so when one currency increases in value, it strengthens against the other, which in turn decreases the other's value. Trading on margin can dramatically amplify these positive and negative changes.