Basis = cash price - futures price. It captures everything the futures contract does not: freight to the delivery point, local supply and demand, grade differences and storage. A hedger who sells futures has not fixed their price; they have fixed the futures leg and are now trading basis.
Basis is usually far more stable and more predictable than flat price, which is why grain merchandising is largely a basis business.
Example: an Iowa elevator bids $4.10 for corn while December futures are $4.45. Basis is -35 cents. If the farmer hedges and basis later narrows to -20, they gain 15 cents even if futures never move.
Related: basis-risk, convergence, cash-market, cost-of-carry