Backend Dev 101
Backend Developers write scripts for servers. Unlike Frontend Developers, the code Backend Developers write, while it gives a lot of functionality to a site, it isn't as visible as the elements built into the HTML. And when you view source, you won't find the backend code in the HTML.
Most of the time Backend Devs write their code for the servers that host websites. These scripts tell the server what to give the browser when it comes asking for things. You can imagine this conversation between a browser (often refered to as the "client") and a server.
This type of conversation runs the gammit from handing off the HTML for a site to storing information in databases and giving it back when it's requested (like when you log in to DIY and it pulls up your account, your favorites, your followers...).
Visit the Node.js download page and download the package that matches your computer's operating system. Follow the install prompts when running in the installer, it will install like any other program.
You can double check your install was sucessful and see what version of node you have by opening your Bash and type:
It should return your version number to you.
All websites have frontend and backend development involved in their coming to be. These two ends togeter are referred to to as a website's stack. If you are a full stack developer then you can do front end and backend development.