Crash rate
Crash rate is the share of people or sessions in which your app crashed; crash-free users is the share who never hit a crash.
Also called Crash-free users, Crash-free sessions.
In plain words
Fern crashed for 3 people on version 2.4, all on the plant photo screen.
How Apparent would say it about Fern, an example app in the live demo.
How it's worked out
People who had a crash ÷ people who used the app. Crash-free users is 100% minus that.
Good to know
Count by person, not only by crash. One person crashing twenty times is one unhappy person; twenty people crashing once each is a bigger problem.
Google Play flags apps where more than 1.09% of daily users hit a crash and can show them less in the store. Apple shows crash counts in App Store Connect and Xcode, but only from people who share data with developers.
The useful details are which version, which screen and which device. A crash that started with the latest version points straight at what changed.
Where the number comes from
Your app's SDK (crashes it catches); Google Play Console (Android vitals); App Store Connect and Xcode.