What exactly is metabolism?
How much food you need to eat in a day to stay at the same weight.
This differs from person based on how big they are or how active they are, for example.
Every single answer in this thread until now is wrong.
What metabolism is, _exactly_:
The chemical _reactions_ in your body necessary to keep you alive. Mostly with the goal of converting what you take in into the stuff your body needs (like energy and manageable waste).
Metabolic reactions can be _anabolic_ (building up stuff) or _catabolic_ (breaking down stuff).
Metabolism is the process by which your body converts the energy bound up in the complex structure of the molecules in the food you eat into energy and resources your body needs.