Margin is expressed as the percentage of the full amount of the position. For example, if the margin requirement is 10% (i.e. leverage of 1:10) and you open a position of $10,000, the amount you will need to deposit is $1000.
The amount of money available in your account to open new positions with is the free margin. For example, if your equity (total amount of money in your trading account, incl. profits and losses) is $11,000, and your open positions require $1,000 (called used margin), you will have $10,000 at your disposal for opening new positions.