Basis is the difference between the spot price and the futures price of a specific commodity at a specific time and place. Zero basis means that the spot price is consistent with the futures price. Under normal circumstances, when the futures contract approaches the maturity date and delivery month, the forward factors contained in the futures contract gradually disappear, the holding cost contained in the futures price tends to zero, the spot price and the futures price gradually move closer, and finally the basis tends to zero.
Futures prices and spot prices eventually converge because the supply and demand factors that determine the prices of spot commodities and expired futures contracts are almost the same in actual delivery. The settlement price of the futures exchange is based on the spot price of commodities at the end of the last trading day of the futures market, which makes the basis tend to zero, otherwise there will be risk-free arbitrage activities.