When a trader places and order that is open for the day. If the price is not hit during that day the order expires.
Other Trading Basics
Backwardation occurs when a bid price exceeds the ask price.
This usually occurs when stock is suspended or under a share repurchase scheme.
It can also mean that a futures contract will trade at a higher price when it is coming close to expiring.
The opposite of backwardation is known as contango.