What's the definition of a proper noun?
A proper noun names something, someone, or somewhere.
John Smith is a proper noun.
A man is a common noun.
Mount Wilson is a proper noun
a mountain is a common noun.
Washington DC is a proper noun
The city is a a common noun.
A proper noun is specific and always capitalized. They denote specific places, people, or things.
Examples of proper nouns:
Thomas Paine (a specific person)
Legend (title of a specific book)
"Ghostbusters" (title of a specific song)
Seattle (a specific city)
Zhejiang (a specific province)
Afghanistan (a specific country)
Muslim (a person of a specific faith)
Islam (a specific faith)
Examples of common nouns (general, not capitalized):
person (could be anyone, not specific)
book (could be any book, doesn't tell which one)
song (could be any song, not referring to a particular one)
religion (could be any religion)