What do numbers in red circles mean on launcher icons? What do these numbers mean on Mac?



I am on an iMac running Catalina.
The number on App Store icon indicates the number of available app updates. The number on the System Preferences icon depends on what option in System Preferences also shows the number. Often it is that there is a software update to macOS available.

In some apps, it's the number of notifications: like email, or messages.