Batting Average
How often a hitter gets a hit when he puts the ball in play or strikes out.
What it actually is. Hits divided by at bats. It is the oldest number in baseball and the one the game was taught with for a century, which is why it still feels like the headline even though it is the least informative of the three numbers on a slash line.
What a scout reads off it. Average tells you how often a hitter got a hit and nothing else. It cannot see a walk, so a patient hitter and a free swinger with the same average are not the same hitter at all, and it treats a bloop single and a ball off the top of the wall as identical events.
Where the lines are. The round numbers the game grew up on still work as landmarks: .300 is the traditional line for a good hitter, .250 is ordinary, .200 is the Mendoza line and the point where a bat has to be carried by a glove. Nobody has hit .400 in a full season since Ted Williams in 1941.
How it lies to you. Average moves around far more than skill does, because whether a ground ball finds a glove is mostly not up to the hitter. A hitter fifty points off his career line is usually the same hitter having a different kind of luck, which is what BABIP is for.
At Little League and travel ball. At Little League level, average is inflated by fielding rather than hitting: a much larger share of balls in play become hits because the defense is twelve years old. Comparing a .500 Little League average to anything in the professional game is comparing two different sports.
Batter's Eye: look up anything in baseball