Strength of schedule is calculated from the opponents projected fantasy points allowed for the DEF 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.
There's a real argument for the Seahawks pushing for the top spot in Team Defenses this year, on paper at least. It's a team loaded with talent and depth at every position. Their best pass rusher Chris Clemons will likely be lost for a period this season, but that hasn't stopped Pete Carroll and Seahawks GM John Schneider from loading up. One of the safest Team Defense options around this year.