UID means unique identifier
So it is a series of digits (and letters) that are unique in a particular setting. It is a random number based on a number of parameters. A Global Unique identifier (GUID) uses hardware information and the date and time of the generation. This results in a combination of numbers and letters that are unique throughout the world.
UID also means User Interface Design
A Dutch translation could be user interface. It is a representation of the operation of a program/website/etc. It shows which actions are stuck, what happens when you press that one button, and what kind of information you get to see. Also in what way it is structured.