Server-side refers to operations that are performed by the server in a client–server relationship in computer networking.

Typically, a server is a software program, such as a web server, that runs on a remote server, reachable from a user’s local computer or workstation. Operations may be performed server-side because they require access to information or functionality that is not available on the client, or require typical behaviour that is unreliable when it is done client-side.

Server-side operations also include processing and storage of data sent from a client to a server.

Server-side @ Wikipedia