HTML is all about structure and content. CSS is all about making things look good. Between the both of them, we can create some pretty nifty looking things:
Despite the niftiness of how sites built using only CSS and HTML look, they will be pretty static. They don't adapt or react to what you are doing. With those two, it's almost like watching a rerun of a great Seinfeld episode over and over again. It's fun for a while, but it gets boring eventually. The web today isn't static. The sites that you use often have a certain level of interactivity and personalization that goes well beyond what HTML and CSS by themselves can provide:
Don't worry if you don't know what any of that means. Just pay attention to what the code looks like. Notice that we see a lot of English words like function, if, else, alert, name, let, etc. In addition to the English words, we also have a lot of bizarre symbols and characters from the parts of our keyboard that we probably never notice. Well, we'll be noticing them plenty enough really soon. Best of all, we'll also fully understand what everything in this code does as well.