Unique ID is an attribute in the Football Manager database. Every item in the database has one of these unique numbers the distinguish it from all other items. Players, staff, clubs, nations, stadiums, cup and leagues all have Unique IDs attached to them (as well as many other things within the database).
The Unique ID of an item can be viewed in a database editor, such as the official FM editor, or from with the game itself. To view Unique IDs in the game, go to the Preferences screen, go to the "Display & Sound" section and select "Show Unique IDs". The game will now display Unique IDs in the title bar.