Margin is the equivalent of a "good faith" deposit. Margin deposits are set by the exchange and are subject to change with price movement and market volatility.
In futures, there are four margin numbers at work.
The Intraday Initial margin is the amount of free funds that must be available in an account to enter the trade.
The Intraday Maintenance margin is the value of the account that must be maintained to hold the position during the session.
The Overnight Initial margin is the amount of free funds that must be available in an account to hold the trade overnight.
The Overnight Maintenance margin is the value that the account must remain above to continue to hold the position.