It means that someone has filed a renewal application. Whether it is a valid renewal depends on a number of things, includindg the right of the "renewer" to claim copyright. For example a copyright cannot be filed in the name of a deceased person. The estate of such person could claim copyright, however, assuming they were the legal representatives etc. If a book has not been renewed the illustrator could claim copyright for his illustrations etc. They might even have precedence over the author's estate. The legal niceties are enormous.