If the curve is in backwardation, a long holder sells an expensive expiring contract and buys a cheaper deferred one, banking positive roll yield. In contango the trade runs the other way and roll yield is negative — a steady bleed even if spot never moves.
Roll yield is the main reason long-only commodity funds and volatility products diverge so badly from the spot index they advertise.
Example: front crude $80, next month $81.20. Rolling long costs 1.5% per month, about 18% annualised if the curve shape persists. A flat spot price for a year still produces a large loss.
Related: contango, backwardation, roll, forward-curve, cost-of-carry