The Umpires
Four of them in a normal game, six in the postseason, and each one owns a different set of calls.
What it actually is. A regular season game has four umpires: one behind the plate calling balls and strikes and plays at home, and one at each base. The postseason adds two on the outfield lines. The plate assignment rotates through the crew so each man works the plate roughly every fourth game. The crew chief has authority over the game, including weather decisions and rule interpretations.
What somebody who knows the game gets out of it. The plate umpire's job is the one that gets graded and it is far harder than it looks: he is judging a three dimensional plane on a ball moving at up to a hundred miles an hour with movement, from a fixed position, several hundred times a night. Umpires call borderline pitches correctly around 92 to 94 percent of the time overall, and nearly all of the errors are in a narrow band around the edges of the zone, which is the band framing operates in.
The numbers worth knowing. Four umpires, six in October. Around 130 to 150 called pitches per game. An umpire misses somewhere between eight and fifteen calls a game by the rulebook zone, almost all of them on the edges.
What people get wrong about it. Balls and strikes have never been reviewable, which is why the automated ball strike system and the challenge version of it are being tested. Everything else on the field is either reviewable by replay or a judgment call that is explicitly not appealable, and knowing which is which is most of knowing when arguing is pointless.
At Little League and travel ball. Youth umpires are frequently teenagers a few years older than the players, working alone or in pairs, and the zone will be wider. The practical coaching point is that the zone is a fact about tonight rather than a negotiation, and the team that adjusts to it first usually wins the game.
Batter's Eye: look up anything in baseball