A commoner is someone who isn't part of the royal family or nobility.
In general, when people use this word they usually mean it as an opposite to "noble" or "aristocrat". For instance, if you're talking about a king and his subjects, most of them would be considered commoners. It's not always a negative term; often it just means someone who doesn't have special privileges or titles because of their family background.
A member of the common people who holds no title or rank.
