Strength of schedule is calculated from the opponents projected fantasy points allowed for the TE 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.
UPDATE 5/25/13: Unless you can get Gronk at a deep discount you'll want to pass on him at this point. Rumor has it that no one knows just how serious his back operation is going to be and when he should be expected to return.5/1/13: New England Patriots tight end Rob Gronkowski appeared well on his way to another monster fantasy season in 2013, but a broken forearm cost the 23-year-old the last five games of the 2013 fantasy campaign, leaving many owners high and dry. Setbacks during Gronkowski's recovery have led to a fourth surgery on the arm, which casts some doubt over his Week 1 availability, as does the status of his balky back. As such, there's a measure of risk involved with investing an early pick in Gronkowski, but as he's shown the last two seasons, when healthy his fantasy upside is through the roof.