Energy is a physical phenomenon that can change in form but never changes in quantity. That is, energy never arises out of the nothingness nor disappears in the nothingness.
(Remark: The word "energy" is also used in meanings other than its physical one, this is not trated here.)
All the living beings depend on energy. The most important source is the electromagnetic energy (light) from the Sun, where it is released in the process of nuclear fusion, mainly of four hydrogen atoms into one helium atom. Many plants and bacteria (the photosutotroph organisms) transform the electromagnetic energy in chemical energy via photosynthesis. The energy is stored in form of carbon hydrates and other organic compounds. The animals, fungi and some more organisms (the heterotroph organmisms) need to take in their energy in form of organic compounds, consuming products from plants or other organisms because they cannot turn electromagnetic energy into chemical energy. (What they can, is use it to warm their bodies.)
Everything we do with muscle energy, we finally do with solar energy, taken in with the food (if I move by bicycle, I move with solar energy).
There are also, quite locally in the depths of the oceans, some bacteria which use chemical compounds coming out of submarine volcanoes to build up other compounds like carbon hydrates. Those are the base of the only known nutrition chain inbdependent of the Sun.
The energy we use technically, is also based in first place on the Sun. When we use carbon or petrol as energy sources, we are using fossilized solar energy. A big problem of those energy sources is that they release carbon and so enrichen the biosphere with this element, in the atmosphere primarily as carbon dioxide. (To believe that electricity is cleaner than fossile energy, as some people believe when coming to electric cars, is very naive because the electric energy needs a source and so cannot be cleaner than that source.)
On the other hand, the devices that turn electromagnetic energy from the Sun into electric energy are becoming more and more effective, easier is the transformation into thermic energy (heat or warmth). (Remark: since energy does not coime into being, is not generated, it is not correct to speak of regenerative or renewable energies, wehat is regenerative or renewable, are the energy sources.)
Some people speak of an energy that is everywhere. Well, such an energy can be defined, still it is something purely theoretical of no practical relevance: an energy that is everywhere cannot be used because for using energy there must be a difference in energy.
Neither the remark that mass is energy (E = mc²) is of any practical use for the energy supply because (at least at the currant state of technics) there is no possibility to release this energy under control, it proves even difficult to keep control over the release of nuclear energy.
Many people unfamiliar with physics want to generate energy, for examplke with a hydrogen motor. Where does the hydrogen come from? From hydrolysis. And whoever knows to think in systems (every cybernetic and probably every ecologist) understands that there cannot come out more energy than has been put in. That miraculous energy increase (and at the same time an entropy decrease) in which some unschooled people believe, contradicts the fundamental laws of physics.
Some consumerists who do not want to save energy, postulate a "free energy", which due to some world conspiracy is not used. Thios term "free energy" exists in physics for the Helmholtz energy (a topic too far off from our contemplations here). But neither the physics nor the ecologists ever postulate that "free energy" of the conspiracy theorists. That is an imaginary energy that arises without source (for exampl,e in a magnet system which constantly releases energy without any energy added to the system). This idea is just as childish as to believe that a rabbit can arise out of the nothingness because we have seen it presented by a conjurer. It is understandable that people who do not want to save energy, invent some nonsense to justify that it is unnecessary to save energy. Practice is different from such chiildish dreams, like everyone knows who understands of physics or ecology. We humans are subject to the laws of nature. On the other hand, some people have a different explanation for the "free energy", that it comes from some source till now not used, something theoretically possible although the name "free energy" maybe is not so suitable. However, speaking of an energy source that is not used, does not solve any problem, especially if it serves as an excuse not to save energy.
Some other energy sources are used, such like the warmth of the Earth (in vast extensions in Iceland, for demonstrative purposes on Lanzarote) and the tides (energy which comes from the rotation of the Earth in relation to the Moon and Sun, accompanied by a slow loss of speed of this rotation).