As you exercise, you consume energy measured in calories. When you consume enough to deplete the available energy, and enough to reduce the sugars that convert easily, the body goes after stored energy. That stored energy is largely in the form of fat deposits in adipose tissue. Remember that nutrition is an important part of any exercise program.
But that process takes place non-selectively. You don't get to choose which fat deposits the body converts. That means, you can't 'spot reduce' by working on your abs. The effect is still achievable, but doing abdominal exercises alone doesn't target that fat.
When you focus on the abs, you will build strength in that area, by increasing the muscle mass in those muscles. That's helpful for a number of reasons. It keeps a firm, strong layer of muscle which helps keep the stomach and other internal organs well inside the plane defined by your hips. You get a nice trim, flat look.
Abdominal exercises help in another way, too. Since the abdominals are large muscles, they consume a proportionately larger percentage of energy than, say, your jaw muscles. That means that as you work them, they have to be supplied with more energy to move through the range of the exercise. That burns many calories, resulting in weight loss and fat reduction.
