can someone please explain this to me?
The short version:
The body uses protein (primarily) as the building block for more/repaired infrastructure -- muscle, organs, etc. For energy, it has 3 basic choices: alcohol, sugars, fats. And it will burn them in pretty much that order. Ignoring alcohol, it comes down to sugars (principally fructose or glucose) and fats. When sugars are readily available, the body burns them everywhere. This makes evolutionary sense to me, because they are transitory in nature - fruit is ripe and edible for a brief period only, vegetables somewhat longer, but not really long term. But excess energy can be stored as fat - preserving the ability to function during bad times.
Keeping that fuel tank well filled is an evolutionary advantage - so the body will always burn the short term stuff and save the long term for ... the long term. Control of this is a highly sophisticated biochemical operation, but at a high level, insulin is the driver. Insulin is released in response to the presence of glucose in the blood stream, and one of its effects is to prevent the burning of fats (lipolysis). Thus, to become a fat burner, you must reduce the presence of insulin in the blood stream.
The body will release the fat stores once it is clear the easy sources - sugars - are not only gone but also don't appear to be coming back any time soon. When the insulin level declines, the fat is released and fat burning takes off. The period of transition is called fat adaptation - and when you've made the transition to primarily burning fat, you're "fat adapted."
I've left out a lot of detail. There are many good sources for more information, including Mark's Primal Blueprint and Phinney and Volek's The art and Science of Low Carbohydrate Living.
thank you so much for taking the time to explain that to me