It's in response to a play last year where Tom Brady got away with completing two forward passes on one play against the Rams. His pass was batted back, he caught it, and threw it again to Mike Evans.
Before now, that was ruled an illegal forward pass, same as if you threw one from beyond the line of scrimmage. But that's just a 5-yard penalty with no loss of down. Brady's pass came on 3rd-and-10, so the Rams had to decide between 3rd-and-15 and 4th-and-2, when it really should have been 4th and 10 or worse. Now it will come with loss of down, so it will be more like intentional grounding.