When trading, you are only required to put up a small amount of capital to open and maintain a new position. This capital is called the margin.
For example, if you want to buy $1000 worth of USD/EUR, you don’t need to put up the full amount to trade. You only need to put up a small percentage like $300 depending on the amount and conditions set by your broker.
Think of margin as a deposit/collateral needed to open a position.