A server is a computer that provides services on request to computers on a network. The services are often related to shared resources--such as printers, data files, media files, web pages--and the server is expected to provide them at any time, day or night.
In most library settings, the installation and maintenance of servers is under the purview of an information technology department, either in the library or at a broader institutional level, and rare is the case where a librarian is expected to maintain a server.
All network-based digital audio projects, however, rely on services provided by servers, so anyone managing digital audio project should have a basic understanding of what servers are, how they differ from desktop computers, and what kind of software they run to provide streaming audio services.