Natural electricity means that electricity is created in the natural world. Natural electricity can not be controlled by humans. This type of electricity happens naturally without any human interference. Some examples are lightning and static electricity. Static electricity happens in a big burst.
Human-made electricity means that the electricity is produced or made by humans. Humans use electric generators to make electricity for lights and other purposes. Human-made electricity can be controlled by humans. An example of this is electricity that we use for lights! The current electricity has a constant flow which is more reliable.
Remember, human-made electricity is more reliable to power a city.