Redditor describes the action:

The two teams in the gif are the Arizona Cardinals (white) and the Atlanta Falcons (red).

The Falcons running back (number 44) is hit by the Cardinals defensive end (94) and the ball is knocked loose (a fumble). A fumble creates a situation where is ball is fair game for any player to grab the ball and consequently control possession, as long as the ball does not "go out of bounds".

A ball needs to actually touch the ground outside the line for it to be considered "out of bounds" though. This is opposite of how it is done in soccer (football to you). I had a hard time adjusting to that one lol. In US football, if any part of the ball crosses the out of bounds line and touches the ground it is considered a dead ball and possession is granted to the team that last touched the ball. If the ball does not touch the ground it is considered a "live" ball and can be controlled by anyone. Furthermore, if the player touches the ground out of bounds while controlling the ball, the ball is also considered out of bounds. This is why you see the defender plant his foot just inside the line before leaping to grab the ball and throw it back inbounds.

For anyone who isn't familiar with the rules of soccer, if the majority of the ball goes out of bounds it is considered dead no matter if it is in the air or on the ground.

Anyway, the defender basically saves the ball and throws it back to his teammate thus "recovering the fumble"... all while in mid-air.

Incredible play by the Cardinal defender.