A flat white coffee is a coffee drink that is a coffee drink that consists of espresso with steam milk or microfoam.
The flat white coffee uses steamed milk as well as small, fine bubbles and a glossy or velvety consistency.
A flat white coffee is sort of like a latte but the flat white coffee is topped with a thin layer of flat layer of steamed milk and nothing else.
So basically they take a coffee and add a thin layer of the steamed milk to make it a flat white coffee.
The lattes are made and served with steamed milk with an added layer of foamed milk.
So when you're ordering a flat white coffee you're ordering a cup of coffee that also has a layer of steamed milk on the top which is why it's called a flat white coffee.