Big ole pool of very fast ram that can be accessed by the CPU and GPU. Lets you run larger models. AMD does the same thing with Strix Halo. I have a 128gb machine at home, and have had difficulties running 120b models, but 70b and below run pretty well.

AMD Strix Halo unified memory still requires allocating X to GPU, x to CPU at UEFI.

So not as flexible as apple's unified memory.