TL;DRFutures contracts expire on scheduled dates. Rollover is closing the expiring contract and opening the next month to maintain continuous exposure.
Every futures contract has a defined expiration date. After this date, the contract settles and ceases to exist. Equity index futures expire quarterly on the third Friday of March, June, September, and December.
As expiration approaches, volume in the expiring contract declines and volume in the next month increases. The transition typically happens 1-2 weeks before expiration for index futures.
Rollover is closing your position in the expiring contract and simultaneously opening the same position in the next month. If you're long 2 ES September contracts, you sell 2 September and buy 2 December.
Most platforms offer a calendar spread order that executes both legs simultaneously. This is better than two separate orders because it locks in the price differential.
You pay commissions and spreads on both legs. The price difference between months also matters. In contango, the next month is more expensive (negative roll yield for longs). In backwardation, it's cheaper (positive roll yield).
For equity index futures, the roll cost is typically small. For commodities in steep contango, it can be significant over multiple periods.
Quarterly ES rollover
It's early December. You're long 1 ES December. Expiration is Dec 20, roll day is Dec 12.
On Dec 12, sell December and buy March. Volume has shifted to March, so fills are better in the new contract. After rolling, your position continues seamlessly.
Missing the rollover
You forget to roll and hold past roll day. Liquidity thins as others moved to the next month.
Spreads widen and fills worsen. If you hold to expiration, the contract cash-settles. You've traded in suboptimal conditions for no reason.
Holding an expiring contract too long to avoid roll costs
Roll costs are trivial compared to trading in an illiquid expiring contract.
Not knowing when your contract expires
Put expiration dates and roll dates on your calendar for every product you trade.
Confusing the continuous chart with the actual contract
Continuous charts splice multiple months. Make sure your order targets the correct active contract.