On-base Percentage
How often a hitter does not make an out.
What it actually is. How often a hitter reaches base by hit, walk or hit by pitch, per plate appearance. Every trip to the plate counts, which is why it is the fairest simple description of a hitter.
What a scout reads off it. The first job of an offense is not making outs, and on-base percentage is the only common stat that measures exactly that. A hitter whose on-base percentage sits well above his average is getting there on his own judgment, and that skill holds up in a way batting average does not.
Where the lines are. Roughly .400 is star territory, .340 is comfortably above average, .300 is the point where a hitter is spending outs faster than he is worth. The gap between on-base and average is the walk rate showing through: sixty points is normal, a hundred is a real eye.
How it lies to you. It counts a walk and a home run as the same event. That is a feature when you want to know who avoids outs and a bug when you want to know who does damage, which is why nobody reads it without slugging beside it.
At Little League and travel ball. In youth ball on-base percentage is inflated by wildness on the mound and is often the loudest number on a team's sheet. The useful split is walks taken against strikeouts, not the raw figure.
Batter's Eye: look up anything in baseball