Futures, in a nutshell, is a voucher that is traded now, and this voucher will be used to pick up the goods on the agreed date in the future. There are no ready-made goods for you at the time of trading.
In this market, you can buy or sell. In the process of falling gold price, I sold gold at 1 for X. After the price fell for a period of time, I bought the same amount of gold at the price of Y, which made the number of buyers and sellers equal, that is, I closed my position. At this time, because the price is falling, X>Y, then X-Y is my profit.