I’ve been trying to figure it out but it’s kind of hard when you don’t even understand why you’re using it in the first place.
A child is linked to a parent object. When parent is disabled, child is disabled. When parent is moved, child is moved. Remember children follow parents and parents don’t follow children.
Prefabs are for storing settings, components, hierarchies, etc. in an easily usable way. If you drag an object into the project view you will create a prefab. Prefabs, unlike children, are not essential to most games, but they help to keep things clean and make it easy to transport game objects between computers.
For further help google “3D parenting” (because this concept is not specific to Unity), and see these pages in the docs: