On the very surface level, it’s a way to quickly see who has been the best-trending hitter or pitcher over a 50, 100, or 250 PA span. If you’re someone who has stumbled onto this website and doesn’t want to know anything more about advanced statistics, you can safely look at the top 50 PA xwOBA leaderboard and know that Greg Allen has been hitting the ball extremely well recently, compared to how he did in his 50 PA prior to the current window.
Move down to the 250 PA xwOBA leaderboard, and suddenly you can see who has been a consistently hot hitter over roughly half a season worth of at-bats compared to their previous 250 PA.