I have a build-in alert, that shows when specific data from within a database is retrieved. How can i set of the alert when the app is running in the background? I have looked at declares, but is a little bit of abracadabra to me…

When an app is running in the background it actually isn’t running but asleep.

Only GPS and some other apps are allowed to run in the background.

You can get your app to wake up every once in a while, but it’s the system that will decide when to wake up your app.
This is included in the Entitlements > background modes