If you try to install an Enterprise distribution signed app you might get a popup when you try to run the app the first time, with the title Untrusted Enterprise Developer.
Starting with iOS 9, there’s no option to “Trust” the developer right from the popup.
You can Trust the developer and enable the app to run in iOS Settings:
Open the Settings app on your iPhone or iPad.
Select the General category.
Select the Profile option.
starting with iOS 9.2 the option was renamed to Device Management instead of Profile.
Tap on the Enterprise App option related to the app (the one mentioned in the popup).
Tap the Trust "The Developer's Name" button.
A popup will appear, tap on Trust again.
You should now be able to run the app, and any other Enterprise app from the same developer.