Carry is the theoretical link between spot and futures. Fair futures price = spot + financing + storage - yield. For gold that is mostly interest; for oil it is tank rent plus interest; for stock index futures it is interest minus expected dividends.
When the futures price drifts above full carry, the cash-and-carry-arbitrage becomes profitable and traders sell futures against physical until the gap closes.
Example: gold spot $2,400, financing 5% a year, storage 0.3%. Six-month fair value is about 2,400 x (1 + 0.053/2) = $2,464. Futures far above that would be arbitraged.
Related: convenience-yield, storage-cost, contango, forward-curve