Walk, or Base on Balls
Four balls, and the hitter takes first.
What it actually is. Four balls, a free base, and no swing required.
What a scout reads off it. The walk is the cleanest window into how a hitter thinks. Chase rate is the pitch-by-pitch version of the same thing, and the two together tell you whether a hitter is in charge of his at bats or the pitcher is.
Where the lines are. A walk rate over about one in eight plate appearances is genuine plate discipline; under one in twenty is a free swinger. Barry Bonds walked 232 times in 2004, 120 of them intentionally, which are both records nobody expects to see approached.
At Little League and travel ball. This is the most undervalued thing a young hitter can learn, and the hardest to coach, because a twelve-year-old is at the plate to swing. At levels where pitchers are wild, taking pitches is often the single highest-value skill on the field.
Batter's Eye: look up anything in baseball