There's a lot of different tools and languages you can pick from to use for backend development. We're going to cover the Node.js platform, but it's just one of many different ways to build a backend server.
Node.js is a platform that allows JavaScript to run outside the browser as it's own program. It's a relatively new tool, and was released in 2009. Before node, JavaScript was only used inside the browser to manipulate web pages. Although node is useful for a lot of different things, it's mainly used to create backend servers to power web applications.
Node's website will tell you that it uses an "event-driven, non-blocking I/O model that makes it lightweight and efficient". What this means is that node is good at handling a lot of small tasks at the same time. On the other hand, it's not very good at doing big complicated tasks that require a lot of work. Node is relatively easy to learn and has a lot of powerful tools available, making it a good choice for learning about backend development. With that said, it's not the best choice for every situation and probably isn't a great option if you're building a very large application.
Like we said earlier, we're going to focus on Node for these lessons. If you're curios, though, there's a lot of other great options. You can write a backend in almost any language you want, but some languages are specifically built for this type of thing.
PHP is one of the oldest and most popular languages to use for building a backend. The vast majority of websites (over 80%) use PHP. Although PHP has had some difficulties in the past, it's grown a lot recently and is a very powerful option. PHP applications are typically hosted using the Apache HTTP Server, which is also widely used and available on most hosting platforms.
Ruby on Rails is a framework that suddenly became incredibly popular around 2008. For a while it was considered the coolest language on the block, and although the hype has died down a lot, it's still a very strong option. Rails allows you to rapidly develop an app using the "convention over configuration" motto.
Java is a very popular choice, especially for large companies. Because Java has type-checking, it can save a lot of hassle when debugging large programs. Java is also relatively fast and can handle a large number of users at once. On top of that, it's been around for a very long time and has a lot of helpful tools/libraries surrounding it.