The Wild Card
Three teams per league that qualify without winning a division, and the reason most of the league is still alive in August.
What it actually is. After the three division winners in each league, the next three clubs by record qualify as wild cards regardless of division. They play a best of three Wild Card Series, all games at the higher seed's park, while the top two division winners wait.
What somebody who knows the game gets out of it. The wild card exists to keep more fan bases engaged later, and it works, but it has a second effect that is more interesting: it reduces the value of winning a division and increases the value of simply qualifying. Once a club is confident of a wild card, the incentive to push hard for the division is smaller than it looks, and deadline behavior has shifted accordingly. More clubs buy, fewer sell, and the standings have compressed.
The numbers worth knowing. Six wild card teams in total across both leagues. Wild card teams have won the World Series several times, which is the strongest evidence that the seeding matters less than the format implies.
What people get wrong about it. All three Wild Card Series games are at the higher seed's park, which is the largest home field advantage anywhere in the postseason, and it is still worth only a few percentage points.
At Little League and travel ball. Pool play seeding is the youth version, and the tiebreakers that decide it, run differential, runs allowed, head to head, are exactly the kind of thing families discover on a Sunday morning. The travel section of this site lists them by sanction.
Batter's Eye: look up anything in baseball