Strength of schedule is calculated from the opponents projected fantasy points allowed for the DL position. The best rating is +16 which gives the player the advantage against the defense and indicates an easy opponent, the worst rating is -16 which gives the defense the advantage and indicates a tough opponent.
Justin Smith isn't getting any younger, but if you need an example of his importance to the San Francisco defense consider that Aldon Smith had zero sacks after Justin hurt his arm last year. Smith's three sacks in 2012 were his fewest since 2007, and the 49ers plan to limit the veteran's snaps a bit more in 2013, but Smith's 66 tackles last year show that he's still a capable run defender. Those tackles equal consistent fantasy production.