Futures price is the future price of commodities, and spot price is the current price of commodities. According to the same price theory in economics, the price difference between them, that is, the "basis difference", should be equal to the holding cost of goods.
What is the spot price difference? It is the price difference between the spot market and the futures market. Once the basis significantly deviates from the holding cost, you can do futures arbitrage and use the price difference between the two markets to buy low and sell high. Of course, you should also consider the cost of futures arbitrage.
It should be noted that retail investors generally do speculative transactions and make profits by buying and selling futures contracts. If the spot price difference is found, you can pay attention to the basis arbitrage opportunity. Since both the futures price and the spot price will fluctuate, the basis will also fluctuate during the validity period of the futures contract.
Basis is composed of many factors, such as freight, price, distance, local supply and demand, miscellaneous fees/profits of other trade, etc. Simply put, it represents the price difference between different physical locations.