Metabolism, or the process by which your body converts food into energy, is commonly associated with body weight. Those who are thin are believed to have a "high metabolism," while those who are overweight are believed to have a low metabolism.
The fact is, weight is affected by the amount of calories you take in, versus the amount of calories you burn. If you take in more calories than you burn, you gain weight. It's uncommon for excess weight to be related to a low metabolism.
Basal metabolic rate, or BMR, is a measure of the rate at which a person's body burns energy, in the form of calories, while at rest. People with more muscle and less fat tend to have higher BMRs, which means they burn more calories while "not moving." Those with low BMRs, on the other hand, burn fewer calories and may gain more pounds of body fat over time.
To improve your BMR, increase daily exercise in order to burn calories and build muscle mass. Some supplements, such as green tea, may also help boost metabolism. But beware of products that claim to dramatically increase metabolism to produce weight loss. Chances are, they are not safe or effective.