The organelle responsible for energy production inside a cell is called a Mitochondrion(Plural: Mitochondria). These pill shaped organelles take food and break it apart into water and carbon dioxide. As food is broken down, an enormous amount of energy is created.
Think of the power generators that produce electricity for our cities. As these power generators break up fuel, they create electricity which is captured and transmitted throughout the city.
Within living things, cells which are more active need more energy. As a result, these cells have more mitochondria. In a similar way, bigger cities need more power, and as a result, these cities have more power plants.