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

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 among 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 steps 1 through 3
6. You will get your UDID

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

You should see many more articles and third-party resources that assist with this, and you may also find some videos that walk you through the process.