A futures contract is a forward contract whose terms are standardized so that it can be closed out with an offsetting trade before maturity. Traders deposit margin to trade, and by the last trading day a buyer can close out by reselling and a seller by buying back.
In an example given by Osaka Exchange, buying one Nikkei 225 Futures contract at 15,000 yen and reselling it at 15,100 yen gives a profit of (15,100 yen − 15,000 yen) × 1,000 × 1 contract, or 100,000 yen. Someone who sold one contract at 15,000 yen and buys it back at 15,100 yen loses 100,000 yen (excluding fees).