The Battery
The pitcher and the catcher, the only two players involved in every single pitch.
What it actually is. The battery is the pitcher and catcher taken together. They are the two positions that touch the ball on every pitch, they choose the pitch and location between them, and they are the only two players whose defensive positions are fixed by the rules rather than by convention.
What somebody who knows the game gets out of it. Catching is the most undervalued position by a wide margin, and the reason is that most of the job never appears in a box score. Framing borderline pitches is worth several runs a year to a good receiver, blocking prevents free bases in the exact situations where they are most expensive, and game calling and pitcher management are real even though they are hard to measure. A catcher hitting .240 who receives well can be more valuable than a catcher hitting .280 who does not, and clubs have known this since the framing data arrived.
The numbers worth knowing. A catcher receives roughly 8,000 to 9,000 pitches in a full season. The gap between the best and worst framers is worth around twenty to twenty five runs, which is two full wins from a skill that has no counting statistic at all.
What people get wrong about it. The catcher calls the game in most cases, not the pitcher, and increasingly neither of them does: the sequence often comes from the dugout through the PitchCom device. Shaking off a sign is real but it is much rarer than broadcasts suggest.
At Little League and travel ball. At youth level the catcher is the single highest leverage defensive position, ahead of shortstop, because passed balls decide games. A 10U catcher who can block is worth more runs prevented than any infielder, and the position is usually given to whoever is willing rather than to the best athlete, which is backwards.
Batter's Eye: look up anything in baseball