What is "udid"

UDID is an acronym for Unique Device ID. This UDID is actually a feature of Apple iDevices. It is basically a unique id for a single device that is being fetched by Apple servers every time a user tries to activate the device using iCloud or Setup app. This ID is also used by iTunes to detect the phone or to communicate with it while restoring the iPSW firmware. This unique ID cannot be changed by the user nor Apple, but changing the motherboard will change said ID.