Plex Media Server is the backend application that manages a Plex library, stores metadata, and streams media to Plex clients such as desktop, mobile, and TV apps. In a client-server setup, the server does the work of indexing content, handling accounts, and delivering media, while the client provides the interface for playback.
It matters in cyber security because the server is the control point for the whole media environment. If it has a flaw in authentication, web handling, file processing, or remote access features, an attacker may be able to reach stored content or disrupt service across all connected clients. Defenders treat updates to Plex Media Server as important patching events, then verify that libraries, login flows, and remote connectivity still work after the upgrade. In practice, security teams watch the server closely because a weakness there can affect every device using the library.