The collateral the exchange requires to open one futures contract and hold it overnight.
Margin and leverage. A $5,000 deposit can control a $100,000 position, which is 20:1 leverage. Because the loss is measured on the full $100,000, a 2.5% move against you halves the deposit and brings a margin call, and a 5% move uses all of it.
Initial margin is set by the exchange based on volatility and changes over time. It is a performance bond, not a loan: you do not pay interest on it. Brokers may require more than the exchange minimum, and much less for intraday positions (day-trading-margin).