In general, the word energy refers to a concept that can be paraphrased as "the potential for causing changes", or Energy is a property of matter that makes things happen, or, in the case of stored or potential energy, has the "potential" to make things happen.
By "happen", we mean to make things move or change condition. Examples of changes in condition are changes in shape, volume, and chemical composition (results of a chemical reaction). There are also changes in pressure, temperature, and density which we call a "change of state" in thermodynamics. Phase changes, such as changing from solid to liquid, or liquid to vapour, or back the other way, are also good examples of condition changes. Something happened!

Without energy, nothing would ever change, nothing would ever happen. You might say energy is the ultimate agent of change, the mother of all change agents.

Whenever anything happens or changes there is an energy change. Either energy changes form, as when a generator changes mechanical energy into electrical energy; or energy changes location, as when heat flowing too fast out of your body makes you cold, or heat flowing into a pot of water makes the water turn into steam. You get the idea.
The most common definition of energy is the work that a certain force (gravitational, electromagnetic, etc) can do. Due to a variety of forces, energy has many forms (gravitational, electric, heat, etc.) that can be grouped into two major categories: kinetic energy and potential energy.