Okay, so you know milk, right? Well, milk is made up of different things, like water, protein, sugar, and most importantly, fat.
Now, fat is like the yummy part of milk that makes it taste good and gives it a creamy texture. But different kinds of milk have different amounts of fat in them.
There's whole milk, which has a lot of fat in it. That's why it tastes so creamy and rich. Then there's 2% milk, which has less fat than whole milk but still some. And then there's skim milk, which has barely any fat at all.
So, the fat content of milk is just how much fat there is in the milk compared to everything else that's in there. And different kinds of milk have different amounts of fat in them!