Energy

Energy (What)

Energy is a property of matter that characterizes the net amount of heat plus work that can be gotten for this matter.

Here is a breakdown of the main ideas that are used to describe energy.

Matter: Matter is general term that means anything made of atoms. So matter can refer to a baseball, a liter of gasoline, a lump of coal and so on because these items are made of atoms.
Property: A property is something that depends on the state (current conditions) of matter. Examples: temperature, pressure, density, energy, volume, weight, entropy, color, and so on.
Work: Work in an interaction between matter and its surroundings that can be used to lift a weight, turn the shaft on a pump, run a chain saw, move a car along a road, run an air conditioner, provide electrical power, and so forth.
Heat: Heat is the movement of thermal energy from hot to cold. Examples: heating water in a pan, heating a house, and so on.

Gasoline has energy because it can be used to provide heat or do useful things like power a generator or power a car.
A battery has energy because it can be used to do useful things like running a mobile phone or powering a light.
Water behind a dam has energy because it can be used to turn the shaft of generator and thereby produce electrical power.

Here are some reasons why understanding energy is worthwhile.

This understanding help you invent, design and apply technology products: cars, dams, airplanes, computers, manufacturing plants, and so on.
This understanding helps you solve problems like how to keep electronic devices from getting too hot and how to keep food safe via refrigeration.

Concerns.

Gaining deep understanding of energy requires lots of effort. There are multiple forms of energy (mechanical, chemical, nuclear, electrical, and thermal) each of them has a great deal of relevant information.
To understand energy requires understanding work and heat. There are many details and nuances. For example, work in thermo is much more than force times distance.
Many textbooks communicate the idea that energy cannot be easily defined.