An incomplete pass is any forward pass that is not caught by an offensive player. In the event of an incomplete pass, the ball will be spotted at the previous line of scrimmage for the following down.
An incomplete pass occurs when an offensive player fails to catch a forward pass. In the final two minutes of either half, incomplete passes stop the clock. This is why it is common to see teams only throw when they trail with little time remaining; the stopped clock allows them more time to huddle up. It is for this reason that teams often spike the ball in the two-minute drill as it counts as an incomplete pass and stops the clock. An incomplete pass can result in a penalty if the quarterback is still in the pocket when he throws and there is no receiver in the area--this is called intentional grounding.