Run support per nine innings measures how many runs an offense scores for a certain pitcher while that pitcher is in the game. That number is then set over a nine-inning timeframe. So the stat essentially answers the question, "How many runs of support does a pitcher receive per nine innings?"
RS/9 is an important tool for evaluating pitchers in the context of their records. Often times, pitchers who haven't pitched well have good records simply because they've received solid RS/9. A similar concept holds true for pitchers who have pitched well but have low RS/9; they sometimes have less impressive records despite their effectiveness.
In no way is RS/9 something a pitcher can control. (On the mound, at least. In National League parks, a pitcher can help his cause as a hitter.) Instead, RS/9 is a nice way of adding context to a pitcher's win-loss record. Does a given pitcher's winning percentage seem a bit too high or a bit too low given his other stats? RS/9 is often the culprit.
It's important to note that for this metric, run support constitutes only the runs that are scored for a pitcher while he is in the game. A few other run-support metrics will take into account how many runs a team scores for its starting pitcher over the course of an entire game. In that vein, RS/9 also works for relief pitchers (although those numbers should be taken with a grain of salt, because relievers have such small sample sizes in terms of innings pitched).
Watch: The Dodgers provide Clayton Kershaw with 5.14 runs of support per nine innings, scoring four times during his seven frames of work.
Because pitchers who get better run support tend to win more games, RS/9 can be a useful tool for choosing between two pitchers with similar numbers. A pitcher with a high RS/9 is likely to earn a higher number of wins -- although RS/9 can often be volatile, especially in small samples.
In A Call
"average run support," "support per nine," "runs of support per nine"