Margin is the amount of the money that is used from your Equity to maintain an open position.
When opening a trade, you will need a certain amount of outlay. This is known as margin. The margin is not a cost but is an amount of money that is frozen when you open a position and returned to you once the transaction has been closed.
It's important to know what amount the margin will be so you can evaluate not only the risk itself but also calculate whether the remaining funds will allow you to open additional positions.