Every day, there are many apps that do not make to AppStore. If Apple has not included any app in its store, then it is because of some genuine concerns or the app might be violating Apple's policy. It is really not advisable to install any app outside AppStore.
But if you are highly sure about the source and wish to install an app on your iPhone or iPad from outside AppStore then there is an option in iOS 9 by which you can do it. First install the app from the source.
Any app outside AppStore will be installed on your screen; but once the app is installed, you will not be able to open it; instead you will read something like this:
This means, even if you have installed the app you won't be able to run it. Here is the tutorial on how to resolve this issue.
How to Install Enterprise Apps on iPhone and iPad Running iOS 9
Step #1. Open the Settings app → Tap on General.
Step #2. Scroll down a bit, and tap on Profiles.
Step #3. In the list of “ENTERPRISE APP”, tap on the app you wish to trust.
Step #4. Now tap on “Trust (name of Enterprise App).”
Step #5. You will be prompted with a pop-up to confirm the operation; tap on Trust“.
That's it, exit from Settings app and tap on the app you wish to run; you will no longer be getting any warning message, and the app will run successfully.
How to Delete Enterprise App from iPhone/iPad in iOS 9
In case you want to delete enterprise app from iPhone; Go to Settings → General → Profile → Tap on the App Name → Tap on Delete App.
Just a word of advice, never install any app that is from an unreliable source, as man apps are malicious which may threaten your privacy, as well as might cause malfunctioning of your iPhone or iPad.