Coined after all-star wide receiver Calvin Johnson, it states that the receiver must maintain full control of the ball even after hitting the ground. The ground can not cause a fumble, but it can cause an incomplete pass (if the player has not made a 'football move')
It refers to a play in Week 1 of the 2010 season versus the Chicago Bears where Johnson had two feet down and appeared to control the ball in the end zone for a touchdown. While getting up to celebrate the touchdown the ball appeared to be loose. After a booth review it was reversed and determined that he didn't keep full control of the ball throughout the play. It was ruled an incomplete pass.
The Calvin Johnson Rule is highly debated due the inconsistency and subjectivity of whether the receiver actually maintains control. In the original Calvin Johnson instance it could be argued that he let go of the ball to celebrate the touchdown.
Another controversial instance of the "Calvin Johnson Rule" occurred in the 2015 NFC Divisional game between the Cowboys and Packers. Dez Bryant appeared to catch the ball, get both feet down, and reach for the goal line with the ball and it bobbled loose after hitting the ground. After the play was challenged, it was ruled an incomplete pass.