Session
A session is one visit to your app, from the moment someone opens it until they leave it.
Also called Session length, Time in app.
In plain words
People spent 4 minutes in Fern on an average visit.
How Apparent would say it about Fern, an example app in the live demo.
Good to know
Most tools end a session when the app has been in the background for a short while, often 30 seconds to a few minutes. Switch to a message and back quickly and it's still one visit.
Longer isn't always better. A weather app that answers in five seconds is doing its job. Read session length against what your app is for.
Apple's App Analytics counts sessions too, but only for people who agreed to share data with developers, so its numbers are smaller than what your own app sees.
Where the number comes from
Your app's own events. Also App Store Connect App Analytics, for people who share data with developers.