Margin requirement: It is the amount of money which your broker needs from you to open a position. It is expressed in percentages.
Account balance: just another name for your trading bankroll. It’s the total of your trading account.
Used margin: The amount of money that your broker “locks” in order to keep your current positions open. Of course this money is still yours. You just can’t use it until it is given back to you or when you close your positions or when you receive a margin call.
Margin call: means that the amount of money in your account cannot cover your possible loss.
Usable margin: This is the money in your account that is available to open new positions.
Quite easy, right? It is also very interesting.