Caffè Americano is a type of coffee that is made by mixing hot water with a shot of espresso. Espresso is a very strong type of coffee that is made by forcing hot water through very finely ground coffee beans.
To make a caffè Americano, a barista (the person who makes the coffee) will first make a shot of espresso. They then add hot water to the espresso shot to make the coffee less strong.
The reason it is called "American" is because during World War II, American soldiers in Europe found the espresso too strong for their taste, so they would ask for hot water to be added to their espresso to make it milder. This became known as a caffè Americano, because it was essentially diluting the strong Italian-style espresso to make it more like the milder coffee that Americans were used to.
So, to summarize, a caffè Americano is a type of coffee made by mixing hot water with a shot of espresso, and it is called "American" because it originated from Americans abroad who found the traditional espresso too strong.