OPS+ takes a player's on-base plus slugging percentage and normalizes the number across the entire league. It accounts for external factors like ballparks. It then adjusts so a score of 100 is league average, and 150 is 50 percent better than the league average.
For example, Miguel Cabrera's .895 OPS in 2014 was 50 percent better than the MLB average after being adjusted for league and park factors. As a result, his OPS+ was 150.
(OPS / league OPS, adjusted for park factors) x 100.
Why it's useful
OPS is a solid tool for evaluating a player's performance at the plate, but it does not tell you how much a player was affected by his home ballpark's dimensions or altitude. OPS+ attempts to adjust for those factors to give you a context-neutral number.
This can be a good tool for ranking players who may have switched teams, because OPS+ adjusts for park and league. Otherwise, for fantasy purposes, OPS is a more useful stat.