I’ll compress this as tightly as possible for the benefit of the casual sports fan. “Sabermetrics” is just a term for the organized study of baseball using the traditional means of science and scholarship; it most commonly refers to the study, in particular, of baseball statistics.
There is a predictable, empirically established relationship between (a) the number of runs a major-league baseball team scores and allows in a season and (b) its won-loss record. This relationship is usually called the “Pythagorean expectation” because there’s a sum of squares in the equation. Bill James, who originally discovered the Pythagorean formula, announced a finding in the early ‘80s that teams which do better in the standings than their run totals would suggest will revert to the Pythagorean norm the next year; on the basis of a limited sample he could find no tendency for teams to outperform Pythagoras from year-to-year.
He has now searched again for such a tendency, using 100 years of data, and found it. Beating Pythagoras appears to be a repeatable skill. If this result holds up under peer review, it will overturn (or at least modify, as Einstein did Newton) one of the most widely accepted statistical laws of baseball.