What is the Audi A8 48-volt mild hybrid system and how does it improve fuel efficiency?

I've heard that the Audi A8 has a 48-volt mild hybrid system. Can you explain what this means and how it helps with fuel efficiency?

The 48-volt mild hybrid system in the Audi A8 allows for smoother start-stop functionality, which can save fuel by shutting off the engine during idling periods.

This system also allows for coasting at high speeds, which can further improve fuel efficiency by shutting off the engine and only utilizing the electric motor.