It is simply ability to do work.
If we go back in history, many of the lifting, moving, rolling and many more actions were done by humans themselves. Then, fire was discovered. So, the first fuel was actually plant-based branches, dried wood, etc.
After that, humans begin to explore and extract the wind power and hydro power via mills to grind and for other uses mechanically. The steam engine was discovered later. Coal, petroleum, natural gas slowly came on board in the energy usage equation.
With the development of technology, a lot of work that was previously done by humans has been replaced by machines. Conversion of energy resources into electricity has made the whole process much easier.
The ever increasing world energy demand has placed ‘energy crises’ as an alert that may take place anytime soon. This has prompted the efficient use of energy to take precedence in design and manufacturing of products.
Now, only through innovation and research we can find a sustainable solution for the one and only earth that we have.