A more thorough explanation:

A reexamination certificate is a document issued by the U.S. Patent and Trademark Office after a reexamination proceeding. This certificate confirms that a patent has been reexamined and the claims have been found to be patentable. It also confirms that any claims that were determined to be unpatentable have been canceled. Additionally, the certificate may include any amended or new claims that were determined to be patentable.

For example, if a patent holder requests a reexamination of their patent and the Patent Office determines that some of the claims are not patentable, those claims will be canceled and a reexamination certificate will be issued reflecting this change. Alternatively, if the Patent Office determines that new claims are patentable, those claims will be added to the patent and a reexamination certificate will be issued reflecting this change.