In order for the body to function , it needs energy.
We get that energy from the food and drink we consume.
One measurement of that energy is the calorie
The body needs a set amount of energy ( calories ) each day , to function normally .
Typically for men 2500 kcal daily - for women 2000kcal daily (These values can vary depending on age, metabolism and levels of physical activity, among other things.)
If you consume more energy than the body requires ; then the body will store the excess energy in the form of fat .
If you consume less energy daily ,than the body requires , then the body will have to get the additional energy it requires from somewhere else .
It will therefore go to where the body has stored energy – the fat bank – and take the extra requirement from there .
Continual less daily intake of energy than the body requires, will force the body to return again and again to the fat bank - thus the fat bank will deplete and you will thus lose weight .
That's my simple explanation
No Mystery