Ball
A pitch outside the zone that the hitter did not swing at, and four of them are worth a base.
What it actually is. A ball is a pitch that misses the strike zone and is not swung at. Four of them in one plate appearance send the hitter to first base, which is a walk. The count is always said balls first, so 3-1 means three balls and one strike, which is a good place for the hitter to be.
What somebody who knows the game gets out of it. Ball four being worth a base is the rule that makes plate discipline a skill instead of a personality trait. It means a hitter who never swings at a bad pitch forces the pitcher into the zone, where hitters do damage, and it means a pitcher's command is worth as much as his velocity. The whole modern value of on base percentage traces back to this one rule, and the reason walks were undervalued for a century is that they look like something the pitcher did rather than something the hitter earned.
The numbers worth knowing. A major league hitter walks in roughly eight or nine percent of his plate appearances. Fifteen percent is elite patience. A pitcher walking more than four men per nine innings is in trouble regardless of his stuff, because free baserunners score at a rate that no strikeout total can offset.
What people get wrong about it. Not swinging is not the same as being patient. A hitter who takes because he cannot recognize the pitch will be run over by strikes, and his walk total will collapse the moment pitchers notice. Real plate discipline shows up as a low chase rate on pitches outside the zone paired with a normal swing rate on pitches inside it, which is why chase rate is the number worth looking at rather than walks alone.
At Little League and travel ball. At youth level, taking pitches is a legitimate strategy and everybody knows it, which is why most leagues eventually add rules to discourage it. A 10U hitter who never swings will walk a lot and will also learn nothing. Good youth coaches ask for aggression on strikes early in the count and patience only with two strikes, so the habit that gets built is recognition rather than passivity.
Batter's Eye: look up anything in baseball