A more thorough explanation:

A reissue patent is a type of patent that is issued to correct unintentional or unavoidable errors in an original patent. It may also be used to revise the specification or to fix an invalid claim. The patentee risks the possibility that previously allowed claims may be rejected. It does not change the term of the patent.

For example, if an inventor discovers an error in their original patent, they may apply for a reissue patent to correct the mistake. The reissue patent will have the same expiration date as the original patent.

Another example is if a patent has a claim that is found to be invalid, the patentee may apply for a reissue patent to remove the invalid claim and replace it with a valid one.