The average person needs about 2000 to 2400 calories a day to maintain their current weight. If you need 2000 calories to maintain your weight but you only eat 1600 in a day then you have a 400 calorie deficit for that day. A pound of fat is about 4000 calories so if you keep eating only 1600 calories a day then after 10 days of doing this you will have a 4000 calorie deficit over those 10 days and will have lost a pound of weight. If you continue this pattern eventually you keep losing weight until you reach a weight where 1600 calories is your maintenance. The opposite is true as well if you eat over your maintenance for calories then you will gain weight until your calorie maintenance is the same as your calorie intake over a long period of time.