A server room is a utility space for computer equipment. While most corporations have a large data center they also have smaller rooms on different floors to allow one to connect all of the Ethernet ports in the rooms to switches and routers which are then connected via high-speed network links, sometimes optical fiber lines, to the main data center.
Small businesses also use server rooms for centralizing the data connections plus have their servers in the small rooms or closets. Controlling physical access allows the expensive equipment to be protected with a lock and possibly an alarm. It keeps non-technical personnel out of the space in case they're tempted to load Steam onto the system with lots of memory, fast hard drives, and extra processing power.
Because computer equipment generates heat, the space should have air conditioning and possibly a fire supression system via a halon installation or a hand-held fire extinguisher.