UDID means Unique Device Identifier and contains 40 characters. It is only required for iOS demos. 

Example: 00008101-001435842661231E

If you see the error "App cannot be installed because its integrity can not be verified", it's because your UDID hasn't been added.

How to Find Your UDID with Mac

1. Connect your iPhone to Mac
2. Go to About This Mac
3. Click on System Report and by USB
4. Under the USB 3.0 Bus, you’ll see your iPhone between all the other USB devices connected to your Mac
5. Select your iPhone from the bottom part of the window, and copy the Serial Number (you may need to add a dash (-) after the eighth digit in the serial number).

How to find your UDID with Your iPhone

You can get it here: https://get.udid.io/

1. Click on Install
2. Enter your passcode
3. Click on Install
4. Tap to find UDID
5. Repeat again points 1-3
6. You will get your UDID

There are many ways to retrieve your UDID. If these two methods don't work for you, then you can go to a search engine and search "find my iPhone's UDID". 

You should see many more articles and third parties that assist with this, and you might also find some videos that walk you through the process.