Let's start with one particular player's "season" stat line:
That's a darned good season, right? If I give you the added information that this player was a leadoff hitter who played shortstop, you're looking at a surefire All-Star and perhaps even an MVP candidate. The player: Derek Jeter. But the numbers aren't for any one season during his long career. Those are his career postseason totals.
In other words, for all that Jeter accomplished for the Yankees over his 20 years in the majors, he played what amounts to an entire extra season in playoff games.