A professional voice is a person that sells the use of their voice in content.  Professional voices are the people you hear on radio ads, TV commercials, or a bank's telephone systems.
They also include the voices you hear in cartoons or video games. To become a professional voice requires years of sustained practice to create a vocal persona that is distinct and desirable.
Customers who want to sell products or services, guide visitors or inform or entertain their audience are always willing to pay for professional voices to represent their interests.