A UDID is an acronym used for Unique Device ID. It is a feature of Apple iPhones and their other devices. Each individual Apple device has their own ID that is being fetched by Apple servers.
Why do I need to share it?
When testing a new mobile application on an Apple device we need a UDID from the iPhone, iPad or iPod. So, in order for you to be able to test your new application on your own phone or tablet, you will need to provide us with the UDIDs from your personal devices.