When you first open an enterprise app you've manually installed, you see a notification that the developer of the app isn't trusted on your device. You can dismiss this message but you can't open the app.
After dismissing this message you can establish trust for this app developer. Tap Settings > General > Profiles or Profiles & Device Management. You then see a a profile for the developer under the "Enterprise App" heading.
Tap the profile to establish trust for this developer.
You're then prompted to confirm your choice. Once you trust this profile, you can manually install other apps from the same developer and open them immediately. This developer remains trusted until you use the Delete App button to remove all apps from the developer.
An Internet connection is required to verify the app developer's certificate when establishing trust. If you're behind a firewall, make sure it's configured to allow connections to https://ppq.apple.com. If you aren't connected to the Internet when you trust an app, the device displays "Not Verified" instead. In order to use the app, you need to connect to the Internet, and tap the Verify App button.