Run support is the number of runs a team scores when a given starting pitcher is on the mound. Over the long run, there should be little difference between pitchers in this category, but from year to year, the number can vary significantly, and even members of the same starting rotation can have very different numbers, which can range between less than two runs per game at the low end, to six or more at the high end. It is obviously very difficult for a starting pitcher to post a winning record with low run support, no matter how well he pitches, while a high run support can make a pitcher a big winner in spite of otherwise poor statistics.
Run support is considered to be largely a result of luck, even though to a small extent, a good-hitting pitcher can contribute to his own run support. This is why it is expected that a pitcher who has benefited from particularly good or poor run support in one season will revert towards the mean the next year, other factors being equal. Some pitchers do seem to escape this rule, but they are merely outliers: for example, Brian Meadows had outstanding run support for a number of years, giving him high win totals in spite of mediocre pitching, while on the other end, Jose Quintana has been notorious for having poor run support year after year.