A noun that lost it's amateur status.
What? I figured that you could get a point for originality.
Fine. A pronoun is a part of speech that is substituted for a regular noun to represent a more formal, direct kind of noun.
So, while a "regular" noun (don't put that on your homework, I don't think that that's an official thing) could be something like "man" or "boat," a pronoun could be something like "David" or "Windigo Harbor." Basically, it just replaces the noun to make it a more real, direct, official noun. Generally, pronouns are capitalized.
Examples of pronouns are:
People (Bob, Sarah)
Places (New York, Montana, San Antonio)
Specific names of things (Hoover Vacuum Cleaner, Swiffer Dust Mop, "Chuck," "Shadowmagic")
I hope that this helps at least a little bit. If not, either ask more questions or try to find a website.