The adjective influential means "having much influence", so influential people or things have the power to change other people or things in some way. This word is often used to describe famous people. For example, Mohandas Gandhi was a very influential person, because the things he did and said changed many peoples lives, and many people believe he has even influenced the world.
Influence can be a bad thing too. For example, World War II influenced many people's lives, making it very difficult for them to live happily even after the war ended.
Influence Facts for Kids. Kiddle Encyclopedia.