Margin is a necessary sum you need to maintain open positions. Usually, it equals to 1-2% of the position size.
Leverage and margin are connected. If you want to trade a larger position but you don’t have enough money, you can use leverage. However, the leverage may be used if only you deposit margin required by your broker. More about leverage is here.
Margin requirement is the amount of money required by your broker to open a position.
Usable margin is the amount of money that is still available to open more positions.
Used margin is amount of money that is locked by your broker until you close your current position or get a margin call.
Margin call is used by brokers to warn that you need to close your trade or deposit more money to meet the minimum margin requirement.