The definition of metabolism is a little nebulous, but generally it refers to all chemical reactions that happen in your body. These reactions are necessary for keeping you alive—after all, your cells need energy to stay alive and function properly. To do that, they run on fuel (calories) and produce waste (byproducts). There's no way around it; without some kind of metabolic process happening in your body, you'd be dead. That said, there's a lot more going on than just calories-in/calories-out when it comes to weight loss or gain. In fact, metabolism can be broken down into three main processes: anabolism (the creation of molecules), catabolism (the breakdown of molecules), and thermogenesis (basically heat production). The first two are important because they relate directly to how many calories you burn each day. Thermogenesis, however, isn't directly related to how many calories you burn each day. It's still important because if your body doesn't generate enough heat, then it won't work as efficiently as possible—and that means fewer total calories burned over time.