Everyone has a metabolism. Some have one that is quicker and others have one that is slower. The slower metabolism people tend to be overweight or if they are in good shape, its because they work extremely hard to keep it that way. Well, there are ways to keep your metabolism performing optimally, and to do so you have to eat.
Think of it this way. When you eat breakfast, your body begins to absorb those nutrients to keep it moving almost immediately. This includes keeping the organs functioning, your brain working, giving you energy, and such. Typically, what you eat gives your body those nutrients for a 
few hours. A lot of people will eat about 3 times a day breakfast, lunch, and dinner. Sometimes its less then that with people skipping breakfast altogether big mistake.
So between breakfast and lunch, there might be a time gap of 4-5 hours. What happens is the first 2-3 hours your body utilizes the nutrition you gave it, and the metabolism is kicked into high gear to burn off those calories you just gave it. The next 2 hours, however, your metabolism slows down because most of the energy from the food is wearing off. You can equate this with getting tired and hungry when too much time has passed between meals.When you repeat this cycle several times a day, and multiple it over weeks or years, you are training your body and mind to behave a particular way with food. Typically, the less food you give your body, the more of “survival mode” it goes into, which is essentially to hold onto fat, slows down how the organs work, slows bloodflow, and more. This makes the ability to lose weight much more difficult!