Alvarus Pelagius was a person who lived a long time ago and he was known for having some really interesting beliefs. He believed that people could be really good and not do anything bad if they just tried really hard, even without the help of God. This is called Pelagianism.
But other people thought this idea was wrong, because they believed that everyone is born bad and that only God can make them good. So they said that Alvarus Pelagius was saying something very bad and wrong.
In the end, the people who believed that everyone is born bad won and Pelagianism became considered a heresy.