Strength of schedule is calculated from the opponents projected fantasy points allowed for the DB 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.
Want to know who the best cornerback in the NFL is? According to Richard Sherman it's Richard Sherman. With over 60 tackles, eight interceptions and three forced fumbles last year, the man has a point. There's a bit of risk involved with drafting Sherman given his (relatively) low tackle output, but in scoring systems that reward highly for INTs and passes defensed he's about as good as they come where cornerbacks are concerned.