The House Analogy
Think of it all as a house. HTML is the foundation of that house. It’s the house structure, the walls, the floors, ceiling and roof. Without it there wouldn't be a house. HTML is like a word document that describes the structure of your web page.
CSS is what you use to style web pages. It’s all the decor and furniture inside your house. The paint and the art on the wall, the rugs on the floor, everything you need in order to make your house look like a real house. You can still have a house without any furniture, the same way you can have a site without any CSS but it just won’t look complete.