What we think of as energy,
is defined as the ability to do things. Energy can produce light, heat, sound, and motion. It allows us to walk and our cars to move down the road. Energy exists in many different forms, but falls into two categories- potential (stored energy) and kinetic energy (motion). Energy is constantly cycling back and forth between the two types.
When we refer to the energy that is used in our homes, schools, and businesses we are generally referring to different sources of energy such as coal, nuclear, hydropower, or solar that we can use to do things for us such as turn on a light bulb or bake a cake.
Energy also cannot be lost, it is simply transferred into other forms (Law of Conservation of Energy). For example, the energy (electrical energy from an energy source) that we use to light a light bulb is transformed into light and heat. In energy efficient lighting, less energy is needed to produce the same amount of light because less is being converted into heat.