Walking can be a good exercise, depending on your level of fitness and fitness goals. Walking is exercise in it's most basic form, and it is certainly better than not exercising at all. For many, walking is literally the first step toward weight loss and fitness, and can be a great place to start. Walking will burn some calories, as it does elevate your heart rate to some degree, and can improve your heart and lung function. Walking will also help keep your muscles active, which is another benefit.
With that being said, depending on your weight loss or fitness goals, walking may not be all you need. To see more dramatic improvements in your health, and faster progress toward your goals, you may want to consider something a little more challenging. Participating in moderate intensity, cardiovascular exercise such as running, bicycling, swimming, etc. on a regular basis will yield greater benefits than walking alone. Incorporating resistance training (exercise bands, weights, body-weight, etc.) will speed your progress even more. Consult with a fitness professional to help design a program tailored to your specific needs and goals. Walking may be a good beginning to your fitness future, but might not be your final destination.