An incomplete pass in football is a pass that hits the ground, is thrown out of bounds, is not caught by a receiver, or the receiver does not maintain control of the ball. When a pass is ruled incomplete, the ball is dead and the next down will be played. Spiking the ball is an incomplete pass.
In 2014, Cowboys receiver Dez Bryant hauled in a crucial catch toward the end of a playoff game versus the Green Bay Packers, however, it was ruled incomplete by the referees due to the ball moving after Bryant hit the ground, and began the ongoing discussion of what is considered a catch.
After a few years, the NFL would rule that Dez did actually catch the ball, changing the rulebook to state that movement of the ball does not guarantee a lack of control by the player. The goal of the updated rules is to clearly define what is a catch so that there is less ambiguity in a referee's decision making.