All it does is bump up the security patch version to the latest one. (sammobile.com)
All told, the game has had over 150 patch versions since initial release, not including the forthcoming HD upgrade. (fortressofdoors.com)
One of the requirements of C&C Generals is OSX 10.9.5 or higher, but as long as the computer runs Yosemite or the newer patched version of Mavericks, it'll run without an issue. (macgamerhq.com)