What is a UDID?
UDIDs are identifying numbers that are unique to individual iPhones and iPads (and iPod touch devices, for that matter). They're 40 characters long, and made up of a mixture of letters and numbers.
How to find out your iPhone's UDID:
1. Open iTunes
2. Connect your iOS device to your computer.
3. Select it in top menu.
4. Click on the device serial number to reveal its UDID.