Johnson was drafted by the Jacksonville Jaguars back in 2014, after a solid college career at Central Florida. Johnson only played in six games in 2014, running for just 86 yards on 29 carries, though he did add two touchdowns. The 6'0", 209 lbs. back hasn't played in an NFL game since, being let go at final cuts by the Jaguars last year.
Johnson should compete with Mike James and probably an undrafted rookie or two for the third running back spot, behind Doug Martin and Charles Sims. That doesn't leave him a lot of room: even if he wins that competition, he's not likely to actually get the ball much. Bobby Rainey managed just five carries and 16 catches as the third back last year, after all.
Still, Johnson adds a little bit of offseason depth to a position that could use it.