Theoretically, the futures price is the future price of commodities, and the spot price is the current price of commodities. The difference between the two, that is, the "price difference", should be equal to the holding cost of goods. (The so-called "holding cost" refers to the storage cost of goods plus the interest paid by asset financing, and then deduct the income from holding assets. With the approach of futures delivery, the holding cost will gradually tend to zero, and the futures contract price = spot price+holding cost). Once the basis deviates greatly from the holding cost, there will be opportunities for spot arbitrage.
For example, on April 7th, the contract price of rebar 10 10 was 4,650, the spot price on that day was 4 190, and the spread = 4,650-4190 = 460 (the holding cost of rebar futures is 60-80 per month, including storage fees and transaction fees). If the spread is reduced to what you think is reasonable, such as 200, you can close your position, buy futures contracts and sell them in the spot. Of course, this is only a simple example, and the real arbitrage should be judged by combining the supply and demand relationship of the market and the future market trend.