Two elaborate a little bit more:
A tag is a 3 letter code that unique identifies each country in the game. For example, Athens is ATH, Rome is ROM, Boi is BOI etc.
The game uses these codes to identify countries. So for example an achievement that requires you're playing as Athens might check that the player country's tag is ATH.
What makes it a bit more complicated than just "your tag is your country" is that some country forming decisions change your tag, while others don't. So for example if you form Macedon this might change your tag your MAC, meaning that you'll be eligible for all Macedonia (MAC) specific content, but not for content that was linked to your original tag. On the other hand if you form the Hellenistic Empire that will only change your name and colour, but keep your tag the same - meaning that you can still get all the events etc. from your original tag.
(Don't quote me on the concrete examples, I don't actually know of the top of my head which decisions change your tag and which don't.)