A person who's an introvert is a person who tends to prefer to be alone and not be around large groups of people.
Introverted people are not always afraid to be around large groups of people however introvert people usually like to be alone and avoid large crowds of people when possible.
An Introverted person tends to also focus on their internal thoughts, moods and feelings and are inward turning and usually doesn't need or seek out external stimulation like an extrovert would.
Extroverts like to be around lots of people and get stimulation from being around lots of people.