What is the Automatic Start-Stop function on the Audi A3?

I'm confused about this feature on my Audi A3. Can someone explain how it works? Do I need to do anything specific to activate it?

The Automatic Start-Stop function is a convenient feature on the Audi A3 that automatically turns off the engine when the vehicle is at a standstill, and restarts it once the driver releases the brake pedal. This helps save fuel and reduce emissions while driving in heavy traffic or waiting at a red light.