Server
A computer that runs continuously and answers requests from other computers. All backend code lives and runs on one.
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.
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.