Your metabolism describes how your body uses energy to do everything it needs to do to stay alive, and function to keep us healthy. The energy is burned in the form of calories. Your metabolic rate is the rate at which your body burns calories.
You might have heard some people say that they have a "fast metabolism", and they can eat a lot without gaining weight. Someone might also have a slow metabolism, and gain weight easily. This can be the case for different people, but your metabolic rate can change. You can in fact speed up your metabolism with certain types of exercise.
Your metabolic rate actually changes throughout the day, depending on the foods you eat, what time of the day it is and your physical activity. You can even burn more calories for hours after your workout, in a process called “excess post-exercise oxygen consumption”, which can go on usually for up to 13 hours post workout.