Margin is the minimum amount of money required in your account in order to purchase a currency on credit. The  brokers usually have a very low margin requirements, but very high  leverage ratio. 

Margin is inversely proportion to Leverage.  For example , a $1000 with the leverage of 100:1 ratio yields a margin of  $10 ($1000/ 100). You will get a margin call, if you use 100% of your margin.