What does the "RW" logo mean in a DVD+R?

It's the logo of the DVD+RW alliance, the developers of the DVD+R and DVD+RW formats.

History of the Alliance

The DVD+RW Alliance is a voluntary group of industry-leading personal computing manufacturers, optical storage and electronics manufacturers including Dell, Hewlett-Packard Company, MCC/Verbatim, Philips Electronics, Ricoh Company Ltd., Sony Corporation, Thomson multimedia and Yamaha Corporation. The group seeks to develop and promote a universally compatible, rewritable DVD format to enable true convergence between personal computing and consumer electronics products.

The logo distinguishes DVD+R/RW disks from DVD-R/DVD-RW and DVD-RAM disks, which are competing and incompatible formats.