A more thorough explanation:

The Ex Post Facto Clause is a provision in the United States Constitution that prohibits the government from passing laws that retroactively punish individuals for actions that were not illegal at the time they were committed.

For example, if a law was passed today that made it illegal to wear blue shirts, and someone was arrested and punished for wearing a blue shirt yesterday, that would be a violation of the Ex Post Facto Clause.

The purpose of this clause is to protect individuals from arbitrary and unjust punishment by the government. It ensures that people are only punished for actions that were illegal at the time they were committed, and not for actions that were legal at the time but later made illegal.