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 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.