Regression Toward the Mean
Regression toward the mean is the tendency for scores to average out. In this case extreme scores tend to happen rarely and seem to fall back toward the average (the mean). For example, a golfer with a handicap of 2 averages a score of 73 (for example). This score represents the golfer's average score. On some days he goes wild and shoots a 63 which is awesome, but extreme. Over time the golfer will have many more scores around his average than far away from it as the scores tend to regress toward the mean of 73.