The UDID is a feature of Apple mobile devices and stands for Unique Device ID that is made up of a sequence of 24 to 40 characters. To get your device UDID we recommend you to use this web-based tool: http://get.udid.io/
Here's an alternative way to get your UDID:
Connect your iPhone to your Mac.
Open iTunes app from the Dock or 'Applications' folder on your Mac.
Click the phone icon, and you'll see your device serial number displayed right below your phone number.
Click on Serial Number so that it changes to display your UDID.