I'd mentioned strength of opposition late in the season as a contributing factor to the White Sox' putrid offense. I neglected to follow-up on this point after 2015 mercifully ended but was reminded of it by an article at Baseball Prospectus this morning.
Basically, opponent RPA+ measures the quality of pitchers (weighted by frequency) a hitter faced. It's also park-adjusted. "RPA" stands for runs per plate appearance and, as you probably guessed, the "+" means it's measured relative to league average. 100 is average. Above 100 means the hitter faced "easier" pitchers. Below 100 means the opposite.