Strength of schedule is calculated from the opponents projected fantasy points allowed for the RB 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.
Brown is currently a free agent and at this point in his career he's nothing more than a situational backup. The fact that he's still unsigned at this point in the off-season speaks volumes as to what the league thinks of his skill set. There are better options come draft day.