Calorie deficit

Eating fewer calories than your body expends.Your body taps into your fat reserves to make up the difference which, in turn, leads to a reduction in body fat. (Here's a link to a calculator where you can estimate your daily caloric needs - http://www.bmi-calculator.net/bmr-calculator/)

Body fat is your evolutionary protection against starvation during periods of famine, our ancestors would eat at a surplus during times of plenty and store body fat. During periods of famine the fat would save them from starving to death.