The Server-Side is the Computer that is hosting the Web-Application or website an End-User is accessing. Look at it this way, the computer you are currently using would be a Server, which means it can host a Website that someone can use. When users access your Website, they are normally sending requests through HTTP or HTTP/2 through the Browser to the Server-Side (Your Computer).
The Server-Side then compiles and sends the HTML5, CSS3, and JavaScript back through the HTTP/HTTP2 pipeline to the user who requested the Website. This happens in a lightning time and it is crucial to capitalize on the speed your Server responds to users' requests. We will be looking in-depth into this topic in the near chapters.
The programming language we will be using in this book is C# as our Server-Side programming language. The reason why we will be using C# on the Server-Side is that it is easy to program and has very easy documentation to read. C# has a very forgiving programming environment and is fully supported by Microsoft. There is a huge community on the internet just in case you experience problems with your code.
[Tip]: One of the important things to understand when programming or making a website is never to give up researching and maintain patience.