A more thorough explanation:

The cause-and-prejudice rule is a legal doctrine in criminal law that applies to prisoners who are petitioning for a federal writ of habeas corpus based on a constitutional challenge. This rule requires the prisoner to demonstrate two things:

1. The claim is based on either a new rule of constitutional law that was not available during the state court proceedings or a fact that could not have been discovered earlier despite due diligence.
2. If the constitutional error had not occurred, the prisoner would not have been convicted, and this must be proven by clear and convincing evidence.

For example, if a prisoner is claiming that their conviction was based on an unconstitutional search and seizure, they must show that the search and seizure violated a new rule of constitutional law or that the evidence could not have been discovered earlier despite due diligence. They must also prove that if the evidence had not been obtained through the unconstitutional search and seizure, they would not have been convicted.

The cause-and-prejudice rule is an exception to the procedural-default doctrine, which means that federal courts cannot grant relief on the basis of a constitutional challenge that was not presented at trial unless the prisoner can show good cause for failing to make the challenge at trial and that the trial court's error actually prejudiced the prisoner. Before 1996, the cause-and-prejudice rule allowed federal courts to grant relief on this basis.