The simplest definition of what your metabolism is, is every function of your body requires energy, this energy consumption is your metabolism. So the more your body has to do to survive in its current state, the higher the metabolism.
Whenever you talk to some Gym instructor or your diet doctor, they will tell you that your "Your metabolism is slow, that is why you can’t lose weight fast!”
And they’re probably right! But it is something you have more control over than you think. The common myth is that genetics plays a large role in your metabolic rate. While genetics does impact your metabolism, it really only accounts for about 5% of your overall metabolic rate. [More]