Out
The only currency in baseball. A team gets twenty seven of them and nothing else.
What it actually is. An out is a batter or runner being retired. Three of them end a half inning. There are only a handful of ways to record one: a strikeout, a batted ball caught in the air, a runner tagged, a runner forced out by a throw to the base ahead of him, and a small number of rules violations. Everything else in the game is arranging those.
What somebody who knows the game gets out of it. Once you see outs as the resource, the whole modern argument about strategy falls into place. A team's offense is not twenty seven chances, it is a budget of twenty seven outs, and any play that spends one to gain something has to be worth the price. That is why a sacrifice bunt is a purchase rather than a gift, why a caught stealing is far more expensive than a stolen base is valuable, and why a man who walks is doing something a man who grounds out productively is not.
The numbers worth knowing. Twenty seven outs a side in a nine inning game. A hitter making an out about two thirds of the time is a good hitter, which is the statistic that makes baseball unlike every other sport people play.
What people get wrong about it. Not all outs cost the same. An out with the bases empty and nobody out is worth far less than an out with a man on third and one gone. The run expectancy table is exactly a price list for outs by situation, and a manager who treats them as interchangeable is the manager who bunts in the wrong inning.
At Little League and travel ball. At youth level a large share of outs are gifts rather than earned: dropped fly balls become outs the following pitch, and runners are thrown out running into tags. This is why youth batting averages read high and youth run totals read higher, and why a coach who teaches runners to stop making outs on the bases usually gains more runs than one who teaches swinging harder.
Batter's Eye: look up anything in baseball