In Europa Universalis IV, lots of console commands require IDs and country tags in order to be used. For example, the add core command, requires the province ID of the province you wish to add to your country (or another country) as a core - without a province ID the game wouldn't know which province you wanted to make a core.
EU4 has a useful feature called debug mode. Debug mode shows addition information when you hover over a country, province or other object on the game map including country tags and province IDs. To enable debug mode in EU4, follow these steps:
To turn debug mode off, simply type the command into the console again (it is a toggle command).