Scientifically speaking, "metabolism" is a term that describes all the chemical reactions involved in being a living organism. And while it is true that metabolism is linked to body weight, a slow metabolism is rarely the root cause of excess weight gain.
While there are health conditions that slow the metabolism such as Cushing's Syndrome or an underactive thyroid gland (hypothyroidism), those cases are quite rare. So while it may be tempting to blame your metabolism for weight gain, it is a natural process and your body has many mechanisms that regulate it to meet your individual needs.
Most often, when we talk about metabolism, we are actually talking about what is known as our basal metabolic rate (BMR). And our BMR is better defined as the average amount of calories we expend while simply living our best lives.