The simple concept of calories in vs calories out is explained below.
The concept of losing body fat is very simple to understand. Excess calories make you fat so to lose fat you simply have to burn more calories than you consume. This concept is so simple there are endless amount of ways to illustrate it.
I’ll use this one as an example. Think of your body like a water tank with a supply pipe running into the tank and a distribution pipe running out of the tank.
If the tank continues to get the same amount of water that is going out, the quantity of water in tank will remain the same. If more water is going into the tank than the amount that’s going out, it will get fuller.
If you want to reduce the quantity of water in the tank, then you have to reduce the water supply or increase the water distribution out of the tank so that more water is going out.
Of course calories and the body is very different to water and a tank, but the concept of changing quantity is the same. What goes into a place of storage versus what goes out determines the quantity of the storage item, device facility etc.