Margin is the amount of money that a trader needs to post as collateral in order to be able to open a position.
Unlike leverage, which is expressed as a ratio (e.g 30:1) margin is expressed as a percentage that refers to how much collateral the trader is required to post.
So, for example, a 10% margin requirement requires you to hold 10% of the value of the position in your trading account, which is also the same as 10:1 leverage.