To follow this tutorial you need to be able to put stuff onto a Web page using hand-coded HTML and CSS.  If you can't then check out the tutorials on those first.

JavaScript lets the user interact more with the page.  They can click to see things pop up, watch slideshows, use fancy menus and so on.  JavaScript is proper programming so you need to be a lot more precise than with HTML and CSS.  If you get those two wrong the browser will normally do it's best to show the page anyway.  Get JavaScript wrong and the browser will just avoid it.

The good news is it's easy to get started with JavaScript now that you know HTML and CSS.