It's a possessive pronoun that shows ownership or connection. It doesn't refer to a specific person, place, or thing.
For example:
* Her book is on the table. ("Her" refers to a female person, but we don't know her name)
To understand the difference, consider these examples:
* Common noun: girl (refers to any female person)
* Proper noun: Mary (refers to a specific female person)
Let me know if you have more questions!