A more thorough explanation:

A request for reexamination is a formal process in which anyone, including the patent holder or an infringer, can ask the Patent and Trademark Office to review the validity of an existing patent in light of prior-art references. The purpose of this process is to ensure that the patent is still valid and enforceable.

For example, if a company believes that a patent is invalid because there is prior art that was not considered during the original patent application process, they can file a request for reexamination. The Patent and Trademark Office will then review the patent and the prior art to determine if the patent is still valid.

Another example is if an infringer is being sued for patent infringement and believes that the patent is invalid, they can file a request for reexamination as a defense. This can potentially invalidate the patent and prevent the patent holder from enforcing it against the infringer.