Baserunners per nine innings pitched tells us the average number of baserunners allowed by a pitcher for every nine innings pitched. For the purpose of this statistic, "baserunners" include men who reach on hits, walks and hit-by-pitches. Errors and fielder's choices do not count.
A pitcher's primary goal is to prevent runs, and he does so by keeping runners off base, making this a very useful stat when evaluating pitchers. However, it does not factor the way in which a hitter reached base. (Obviously, home runs are more harmful to pitchers than walks.)
Although this stat can be useful, the baseball world prefers to use its cousin -- WHIP -- which is basically the same thing except "per inning" instead of "per nine innings." Also, WHIP does not factor hit batsmen into its equation.