What is Server
A computer that runs continuously and answers requests from other computers. All backend code lives and runs on one.
In plain terms
When an app loads data, the request travels over the internet to a server, which runs the backend code and sends an answer back. Companies once owned their servers, and today most rent them from cloud providers like AWS, Google Cloud, or Azure. 'Server-side' on a resume is a synonym for backend work.
What it tells you about a developer
Comfort with servers goes beyond writing code for them. Deploying, monitoring, and debugging them live marks the line between shipping features and owning a service. Carrying on-call duty is a clear marker of that kind of production ownership experience.