A more thorough explanation:

The overbreadth doctrine is a principle in constitutional law that states that if a law is written so broadly that it restricts free expression, it can be invalidated even if it also prohibits actions that are legitimately forbidden.

For example, if a law prohibits all forms of protest, including peaceful picketing, it may be considered overbroad because it restricts the right to free speech. Even though the law may legitimately prohibit violent protests, the overbreadth doctrine allows for the entire law to be struck down because it has a chilling effect on free expression.

The Supreme Court has used the overbreadth doctrine to invalidate laws that would require loyalty oaths or restrict the distribution of literature. These laws were considered overbroad because they restricted free expression beyond what was necessary to achieve their intended purpose.