Metabolism is simply the term given to a set of chemical processes that occur within an organism to maintain its continued functioning/life.
Our cells need energy to conduct processes which they get primarily from the food we consume. When we take a bite of food, say a potato, our bodies which are made up of organs (organism), which are made up of specific types of cells (liver cells, skin cells, brain cells, bone cells, heart muscle cells, etc) receive the energy from the potato by performing a set of chemical transformations on the potato.
In brief, that would be, one eats potato, the body secretes enzymes to break the potato into smaller and smaller and smaller components to the point where the potato is reduced down into it is basic molecular individual parts (H20, Glucose, minerals, vitamins, etc, etc), it is then able to enter your bloodstream through absorption across your intestinal wall where it is then transported all around your body by various cells and binding proteins flowing through your circulatory system and delivered to the other cells where needed.
These cells use the nutrients for energy to conduct their specific functions, whether it be making a muscle move, producing more chemicals, repairing damage, growing, replicating, etc, and produce waste products as a result. This is what the cell does continuously to continue living. This is what metabolism is. We as organisms represent this process but on a far greater scale as we are the combination of every single cell in our bodies doing this continuously.
The waste by-products of all these actions are cleared from your body by various mechanisms, for example, when you inhale air, it goes into your lungs where it is filtered down so finely that oxygen can diffuse straight into your bloodstream, which is then shuttled to cells all over your body by hemoglobin (a protein which oxygen binds to in red blood cells so it can be shuttled around your body), where various cells can use it as a component of energy production along with glucose, etc, producing carbon dioxide in the process. Carbon Dioxide is one waste product of cell metabolism which is then bound back to blood cells, where the molecules are transported back to your lungs where it can diffuse across your capillaries into your lungs again and you breathe out the carbon dioxide.
Conceive how you need to breathe continuously to ensure the cells in your body have oxygen to continue their processes and what happens if you stop breathing for too long. This is how fast processes are taking place within your body, and it is quite amazing the grand scale result that emerges from all our microscopic cells coordinating, cooperating, replicating and dying continuously.