Unlike apps from the App Store that can run directly after been downloaded on the iOS devices, there are 3 several steps you need to do to install the ipa file on your device and run the app.Check the complete tutorial.
Tap Install to download the IPA file on your iPhone.
Tap Settings > General > Profile & Device Management; you will see the developer of this IPA file under "Enterprise Apps".
Tap the Enterprise name and then tap “Trust” on the pop up window to confirm your choice.