This is actually a phrase from statistics that has been around for a long time. The basic idea is that you might observe "outliers" around the mean, which in this case might translate to having a season w/ 1200-1500 yards receiving and 10-15 TDs - But over time, as more data over more seasons is collected, you'd expect more average-like numbers (perhaps 800-1000, 6-8 TDs) - That is, you'd expect reversion toward the mean or average ("expected") level of performance. I chose those number for illustrative purposes only, FYI. Hope that helps.