In shortest possible way to describe: the traditional shared memory means a portion of RAM is reserved for the GPU, while in Apple’s unified memory both GPU and OS can access the entire RAM. That speeds up things by avoiding unnecessary data copying to the specific graphics-reserved part of RAM so the GPU can access it, as far as I understood, based on this useful thread: https://news.ycombinator.com/item?id=27182715
The question remains whether current apps and games for Macs can actually utilize this advantage.