die App
app
An application program for a mobile device such as a smartphone or tablet, or for a computer.
Hast du eine gute App, um das Wetter zu prüfen?
Do you have a good app for checking the weather?

app
Loading your saved state.
app
An application program for a mobile device such as a smartphone or tablet, or for a computer.
Hast du eine gute App, um das Wetter zu prüfen?
Do you have a good app for checking the weather?

'App' is an English loanword and a short form of 'application' ('Anwendung' in German). It has become established in German as a common term for computer programs on mobile devices.
Ohne diese App könnte ich mein Handy kaum nutzen.
Without this app, I could hardly use my phone.
In everyday language, 'App' is often combined with certain verbs. You can 'herunterladen' (download), 'installieren' (install), 'öffnen' (open), 'nutzen' (use), or 'löschen' (delete) an app.
Ich muss die neue Version der App noch herunterladen.
I still need to download the new version of the app.
To describe the purpose of an app, German uses 'für' (for). To indicate where it is installed, German uses 'auf' (on).
Ich suche eine gute App für die Navigation in der Stadt.
I'm looking for a good app for navigation in the city.
program
'das Programm' and 'die App' can both mean software. 'das Programm' is the more general term for software or an application, especially on a computer. 'die App' usually means an application program on a smartphone or tablet, but it can also occur on a computer. Learners may confuse the words because every app is a program. But not every program is called an app in everyday language. For phone software, 'App' sounds more natural; for computer software, 'Programm' often does.

Fiona nutzt eine App, um ihre Schritte zu zählen.
Fiona uses an app to count her steps.
Für die Arbeit öffnet Tom jeden Morgen dasselbe Programm am Computer.
For work, Tom opens the same program on his computer every morning.