Margin
Margin is the deposit your broker sets aside to keep a leveraged position open. It is collateral, not a fee.
When you open a leveraged position, part of your capital is set aside as collateral: this is the required or used margin. It is calculated by dividing the position value by the leverage.
That money comes back to you when you close, plus or minus your result. Its main effect is to limit what else you can open: free margin is the money still available.
Don't confuse margin with risk. Low margin does not mean a low-risk trade: if your stop is far away or missing, you can lose far more than the margin held, right down to the stop out.