App Developers require the UDID (Unique Device Identifier) from their clients or team members in order to allow them to test the app on their devices. UDID is the Apple way of authorizing a device to install apps out of iTunes. This is necessary for development teams especially when multiple team members like to test the apps on their approved devices.
In order to approve a device to run an app, we need UDID.
Now, After iOS 7 Apple is not allowing UDID to be accessed with all public APIs.
The quickest way to locate your UDID is by opening the website http://get.udid.io/ in your Safari iOS browser. With your quick acceptance for installing the OTA certificate, in no time you can see the DID, IMEI, and also the Serial Number of your iOS device appearing within your browser window – It’s just two clicks away. You can also scan this QR code to locate the site.
But as a Mobile developer who repeatedly works on several apps, I know you would like to make your own app and distribute it among your devices. There is a detailed set of instruction at StackOverflow.