Leverage
Leverage lets you control a position larger than your capital. At 30:1, €1,000 of margin gives you €30,000 of exposure.
Leverage is the ratio between the size of your position and the margin you tie up. With 30:1 leverage on EUR/USD, a €30,000 position needs only €1,000 of margin, or 3.33%.
It magnifies gains and losses alike. A 1% market move equals 30% of your margin at 30:1. That is the main reason so many retail accounts shrink so fast.
The maximum leverage on offer is not the leverage you should use. What matters is effective leverage (total exposure divided by your capital) and above all your risk per trade, set by your stop and your position size.