Margin (a.k.a. "Used Margin", as opposed to "Free Margin") is the minimum amount that needs to be present in your account in order to be able to execute and maintain a position (an open trade). Margin is directly related to Leverage, for example, with $1,000 in your account and a 1% margin requirement to open a
position, you can open a position worth of up to a notional
$100,000. This allows a trader to leverage his account by up to 100
times or 100:1. Of course fully using up the available leverage is a very bad idea, because in this case you will have absolutely no Free Margin left and any move of your open position against you, even by 1 pip will result in a "margin call"
requiring you to either add more money into your account or close the
open position. Most brokers will automatically close your open
positions when the margin balance falls below the minimum level. The
amount required to maintain an open position is dependent on the broker
and could be 50% of the original margin required to open the trade.

Here's some sample calculations showing the relationship between Margin and Leverage:

0.25% margin = max 400:1 leverage,

0.5% margin = max 200:1 leverage,

1% margin = max 100:1 leverage,

2% margin = max 50:1 leverage.