Downloads
Downloads count how many times people got your app from the store; first-time downloads count new people, redownloads count people getting it again.
Also called First-time downloads, Redownloads, Installs, Units.
In plain words
2,431 people downloaded Fern in the last 30 days, and 312 came back for it again.
How Apparent would say it about Fern, an example app in the live demo.
Good to know
Apple separates first-time downloads (someone's first time getting the app on their Apple Account) from redownloads (getting it again, say on a new phone or after deleting it). Updates aren't downloads at all.
Google Play counts installs per user and per device, so one person with a phone and a tablet can be one user install and two device installs.
A download isn't a user yet. Some people never open the app. Compare downloads with first opens from your app to see how many really start.
Apple's sales reports call downloads "units". They include free and paid downloads and arrive a day later.
Where the number comes from
App Store Connect sales and analytics reports; Google Play Console statistics.