* Common nouns refer to general, non-specific people, places, things, or ideas. They are not capitalized unless they start a sentence.
* Proper nouns refer to specific people, places, things, or ideas. They are always capitalized.
"Playing tag" is a general term for the activity of chasing and tagging others. It doesn't refer to a specific instance of playing tag or a specific group of people playing tag.
Let me know if you have any other questions about nouns!