Here's why:
* It names a thing: It refers to a specific type of entity (nations).
* It can be the subject of a sentence: "Countries" can be the thing doing the action in a sentence. For example: "Countries are working together to solve climate change."
* It can be the object of a verb or preposition: "I visited several countries."
Let me know if you have any other words you'd like to identify the part of speech for!