Your website is made up of 2 primary types of code: HTML and CSS.
Imagine building a house. You start with the foundation, then you put up the frame of the house. After that, you add your walls, floors, and ceilings.
That's HTML. It is the foundational elements of the house.
Now go back to your imaginary house. Pick the color you want to paint the walls, the backsplash pattern for the kitchen, and the flooring throughout the house.
That's CSS. CSS is the interior designer that says "Hey, I'm going to make this look beautiful!" And just like with a painted wall, you can always change your mind with CSS without affecting the structure of the site. You just pick a new color and you're done.
Now, this is a simplified look at what each coding language is. HTML can do design things that CSS does, and CSS can do some structural things HTML does. But generally, HTML is the structure and CSS is the design.