These three tools are usually grouped together because they are all necessary to build interesting websites, but each one totally unique. You'll have lots of time to learn all about each one later. For now just getting the idea is enough.
JS is a programming language, it is the meat in your cake. JavaScript is a full programming language with super powers. Along with the standard functionalities of any programming language JavaScript can:
- Store and manipulate user data (make pages interactive)
- Manipulate the DOM in your browser (change what people see)
- Send and receive HTTP requests (talk to other computers)
- Offload work to 3rd party services (call API's)
You'll learn more about all these features later on in this bootcamp. For now you'll just have to worry about the basics (functions, variables, control flow), and a little bit of DOM manipulation.
HTML is a markup language used to define layout and appearance of your website. At least until HTML5 came out. HTML5 adds a lot of functionality to your markup, but is still not as powerful as JavaScript or CSS.
CSS isn't quite a Markup Language, and isn't quite a programming language. But it can be used to many of the same things as JS or HTML. CSS is it's own strange beast, some love it and some hate it.