This unit is about the way we get usable energy.
Firstly, we can use fossil fuels - coal, petroleum (crude oil) and natural gas, pretty much as we do now. They have good and bad points:
Cheap, you have to find them, but not make them.
Relatively easy to obtain (dig a hole, drill down a kilometre or so, etc.)
Easy to carry & use, especially the liquid fuels
Give lots of energy in a small volume
They produce lots of carbon dioxide, a factor in climate change
They create other forms of pollution, like acid rain and soot
They spoil the environment
They are going to run out soon
You can’t turn power stations off at night or when you need less power
Fossil fuels are only fit for burning, so if you want electricity or movement, you have to use complicated machines that waste lots of energy as heat.
Nuclear fuels give lots of cheap electricity, with little pollution, but the waste is poisonous and will be dangerously radioactive for tens of thousands of years.