Simple Definition:

An introvert is someone who tends to be more inwardly focused and may find socializing exhausting or draining. They prefer to spend time alone or in small groups, and may feel more comfortable in quieter, more low-key [modest or restrained] environments. For example, an introvert may choose to read a book or watch a movie alone at home instead of going to a party with lots of people.

Very Simple Definition:

A person who likes to spend time alone and feels happy when they are alone, rather than when they are with other people.
Example: Lisa is an introvert, she enjoys reading books by herself rather than going to parties.