The share of swings that miss — a per-swing measure of bat-missing stuff or swing-and-miss tendency.
Whiff rate is swings-and-misses divided by total swings. For a pitcher it measures how hard the arsenal is to make contact with; for a hitter it measures swing-and-miss tendency, often broken out by pitch type.
Because it is measured per swing rather than per plate appearance, whiff rate is a cleaner read on stuff and contact ability than strikeout rate, which also depends on how at-bats end.
Pitch-type whiff rates drive the arsenal-versus-hitter view: a pitcher whose best whiff pitch attacks a hitter's weakest pitch type is a strikeout-prop angle.
For a pitcher, yes — more misses per swing means better bat-missing stuff. For a hitter, a high whiff rate is a weakness that raises strikeout risk.