What is the Italian rule for espresso?

Excuse me, could you please elaborate on the Italian rule for espresso? I'm quite intrigued to know the specific guidelines or standards that Italians adhere to when it comes to preparing this beloved beverage. Is it about the ratio of coffee grounds to water, the temperature of the water, or perhaps the pressure used during the extraction process? I'm eager to learn more about the nuances that make an Italian espresso truly authentic.