What is unified memory on Apple devices?

Apples memory architecture uses the same memory for CPU, GPU and Neural Network operations. As opposed to architectures that have separate memory for the GPU, for example.

The memory manager can dynamically allocate the entire memory space across all the SoC components.

When CPU, Graphics processing Unit, and Neural Processor all have access to the same memory, it can be allocated wherever it is needed. It's almost as if you have as much of any kind of memory you need for any reasonable task.

Also, not having to have a separate Graphics memory means that you never end up tediously copying stuff back and forth to work on it with a different processor. That provides a performance increase in itself.

Another side effect is that the memory has so much bandwidth, you almost never have to wait for things from memory, so everything runs faster.

The Bottom Line: Much faster operation overall.