A more thorough explanation:

The new-rule principle is a legal doctrine that applies to criminal procedure. It prevents federal courts from granting habeas corpus relief to a state prisoner based on a new rule that was announced after the prisoner's conviction and sentence became final. This means that if a new rule is created after a person has been convicted and sentenced, they cannot use that new rule to challenge their conviction or sentence.

For example, let's say that John was convicted of a crime in 2010 and sentenced to 10 years in prison. In 2015, a new rule was created that would have made John's conviction invalid. However, because John's conviction was already final, he could not use this new rule to challenge his conviction.

Another example is if a new rule is created that changes the way evidence is admitted in court. If a person was convicted before this new rule was created, they cannot use this new rule to challenge their conviction.

These examples illustrate how the new-rule principle works. It ensures that people cannot use new rules to challenge their convictions or sentences if those rules were not in place at the time of their conviction.