If you think the futures price will go up, go long (buy and open positions), go up (sell) and close positions, and earn: price difference = close positions-open positions.

If you think the futures price will fall, short (sell the position), fall (buy) and close the position, and earn: price difference = opening price-closing price.

The seller is short (signing a selling contract with others). If the futures price falls, the seller will buy the futures at a low price (current price) and hand it over to the buyer at the contract price (buy and close the position) to make money; However, if the futures price rises, the seller will be forced to buy the futures at a high price (current price) and give it to the buyer at the contract price (buy and close the position), so it will lose money.