A runner is said to be in scoring position when he is on second or third base. The distinction between being on first base and second or third base is that a runner on first can only score if the batter hits an extra-base hit, while a runner on second or third can score on a single.
Many of baseball's "little ball" or "one run" tactics center around attempts to move a runner on first base into scoring position. Such tactics were dominant before the 1890s and during the Deadball Era, when extra base hits were relatively rare. The modern prevalence of power hitters has made the distinction less important, and some announcers will say that "first base is scoring position" when a notable power hitter is batting.
One measure of clutch hitting ability is how well a batter hits with runners in scoring position, as a high batting average in such situations will result in a large number of RBIs. While there are significant differences in this statistic among batters, leading to different results for the batters' teams, the ability to hit particularly well in such situations has never been shown to carry over from year to year, leading sabermetricians to conclude that clutch hitting is largely a matter of luck.