"Server-Side" means the server side (or backend end) development phase of a given project.
For example, technologies such as Asp.NET, Java, LINQ, Entity Framework, PHP, and even Python are used to develop the sophisticated middle-tier aspect, often handling the connectivity of databases, security (such as data validation and SSL), handling processing and much more!
I have over 13 years experience with server-side technologies, using the said above technologies and many more. Conversely, my primary server-side technology of choice is Asp.NET 2.0-Core over any other server-side technology, due to my in-depth experience in Microsoft technologies. In fact, my experience across all business tiers of a modern web or mobile application is why I can offer such affordable rates and provide so much support.