It’s a chip which replaces the controllers (previously several of them, from different vendors, that Apple didn’t have control over) for disk and network I/O, speakers, microphone, camera, cooling system, etc. with a single Apple-designed chip. This lets Apple add features to their hardware unsupported by other controllers, move some features from the CPU to this new chip which can reduce attack surface area and free up CPU cycles for something else, save space on the motherboard, and maybe save money.
