My understanding is that the term no longer has a specific legal meaning but in general usage it means that although the author is permitting you to use their property as specified they are not thereby relinquishing their ownership of any rights attached to their ownership. This is/was meaningful as relinquishing of rights by the owner through non-enforcement was/is a legal defense used by those charged with copyright infringement. The classic case is Kleenex which did not adequately enforce the ownership of their trademark for tissue paper while Xerox did enforce the protection of their trademark for photostats. This is why some people/organizations such as Matel enforce copyright in cases of "Fan Art"... they do not want to permit a dilution of their enforcement of copyright/trademark that might be used as a defense in cases of real commercial enfringement. Enforcement is like building a fence... sating "All Rights Reserved" is like posting a sign on the property line so people can not claim that you have abandonded your property rights.