The simple answer is, this is an espresso drink with a small a layer of frothy microfoam made from steamed milk.
A coffee with milk added to it was called a white coffee. Since there is a very small layer of flat foam (or foam with no bubbles) in it, it was called a flat white!
the milk and espresso offer a nice balance between bitter and sweet. It also has distinct nutty and chocolate taste.
A regular flat white coffee contains two espresso shots, which gives it a grand total of about 130 milligrams of caffeine