It took me a while to understand too.
In a nutshell, we train our bodies to use fat as a fuel instead of carbs. By eating less than 20 grams of carbs per day, we can usually get into ketosis within a few days. Ketosis just means that we are mostly burning the fat we eat for fuel.
Fat adaptation can occur within 2-3 weeks, longer for others. This is when the body has learned how to take the fat from our bodies, as opposed to the fat in our bellies, when it needs fuel… So if you lower the amount of fat you are eating during fat adaptation, the body has no choice but to use the fat on your body. When you can go 24 hours or more without experiencing hunger and without decreased energy levels, you are there.
Fasting is something you can do after you are fat adapted. You can go for a day or longer without eating because the body has become efficient at taking the fat off our bodies, in order to fuel itself.