Many futures contracts have daily price limits, equally higher and lower than the previous day's closing price. When a market moves to the daily price limit, it is said to have made a "limit move." This is also referred to as limit up or limit down.
Similarly, many futures options contracts have daily price limits, which are most often equal to the daily price limit of the underlying contract.
During limit moves in futures, traders will often use the related option contract to determine the implied futures price, or where the futures would be trading if there were no limit. Traders will use synthetic futures contracts, a put and a call at the same strike price and same month, to determine the implied futures price of a limit move. For example, with December corn futures locked up the limit at $4.00, if the trader were to buy a December corn $4.00 call option and sell a December corn $4.00 put option, the difference between the two prices would give the trader the implied price where the futures were trading.
If the $4.00 call is trading at 25 cents and the $4.00 put is trading at 20 cents, then by adding the difference of 5 cents to the strike price, the implied futures price would indicate a $4.05 during a limit up move to $4.00.