It starts with Sir Francis Galton and his 1885-86 study of heights. Very tall parents tended to have tall children, but on average those children were a bit closer to normal height than their parents. Very short parents had children who were a bit less short. The extremes pulled back towards the middle, and statistics has used the word "regression" ever since.
Once you see why, it's obvious. Every result is part skill and part chance. Whoever tops a test on day one is good, but probably had a lucky day as well. On day two the skill turns up again and the luck mostly doesn't. The more luck went into an extreme result, the less likely that result is to repeat.
The whole rule fits in one line:
expected next value = average + r × (observed value − average)
r = how much of the spread is real, repeatable skill
r = 1 → nothing regresses, the result was all skill
r = 0 → straight back to the average, the result was all luck
Football sits somewhere in between. For some measures, finishing luck above all, r is much closer to 0 than most people assume.
The jinx that never was
Sport is full of superstitions that are really regression in fancy dress. The Sports Illustrated cover jinx is the famous one: an athlete makes the cover after an extraordinary spell, then has a more ordinary one, and the magazine gets the blame. Baseball has the sophomore slump, where a brilliant rookie season is followed by a duller second year. Nobody cursed anyone. The cover and the rookie award were both handed out at a peak, and peaks don't last.
The flip side is the regression fallacy, where whatever you did at the low point gets the credit for a recovery that was coming anyway. Speed cameras go up after a spike in crashes, the crashes fall, and the cameras take the bow. Football's version is the manager sacked after a terrible run. Hold that thought; we'll come back to it.