Have you used something like sass to create your css files? if so, css variables are the same idea, only now you can do it natively in css. You don't need to create a separate sass file, styles.scss, and compile it to a styles.css file.

Instead you can define values for variables directly in your css file, then use those variables further down. In a traditional css file, you might have a "red" color that you use for a bunch of things. In the css file, you'd have the value for that color, like '#ff0000' hard-coded everywhere. If you wanted to change this red color to pink, you'd have to do a search and replace throughout the css file, making sure to only change the relevant styles and leave any others that just also happened to be red alone.

With css variables, you can define a variable like --main-highlight-color: #ff0000. Now you can use var(--main-highlight-color) in your css file wherever it applies. If you want to change that color, you just change it in the definition, and you'll see the change in any style that uses it.

Name a thing:
--something:red;

Use it anywhere:
background:something;

Change the speed of this animation by altering the "length" at the top of the CSS:
codepen.io/vuild/pen/EGwKGX

Look through the CSS. Anywhere it says "length" will be replaced by that variable.