A more thorough explanation:

Definition: CPC stands for Certificate of Probable Cause. It is a document issued by a United States circuit judge in an appeal from the denial of federal habeas corpus relief. The certificate certifies that the prisoner has shown that a constitutional right may have been denied. The prisoner does not have to show that the case would succeed on the merits, only that reasonable jurists would find the claim at least debatable. If the certificate is not issued, no appeal is possible.

Example: John is a prisoner who has been denied federal habeas corpus relief. He believes that his constitutional rights have been violated. He applies for a CPC, which is issued by a United States circuit judge. The certificate allows John to appeal the denial of his federal habeas corpus relief.

Explanation: The example illustrates how a CPC works in practice. John is able to appeal the denial of his federal habeas corpus relief because he was issued a CPC. The certificate shows that John has shown that a constitutional right may have been denied, which is enough to allow him to appeal the denial of his relief.