Energy is the ability to do work. There are two kinds: Potential and Kinetic. Potential Energy can do work when something happens, like when you push a rock off table. Kinetic Energy is the work that happens when something is already moving. Work is defined as moving something that has mass, so if you push a pencil that is work. The Energy is what allowed you to move the pencil. That energy came from the food you ate.
Einstein's theory states that matter can be converted to energy and energy can be converted into matter. We have created matter in labs from energy to prove this concept. All the matter in the universe is believed to have been created by this process too, known as the "Big Bang."
Nothing is destroyed in the process, since you are just changing the energy into matter by rearranging things.
