Source: Networking—A Beginner's Guide, 5th Edition by Bruce Hallberg (2010)
Example:
Network security is provided by a combination of factors, including features of the network operating system, the physical cabling plant, the network connection to other networks, the features of the client workstations, the actions of the users, the security policies of management, and how well the security features are implemented and administered.
what exactly is that?
refers to the physical cables and connectors which run throughout an installation and can also include the external cables entering into the installation.
An example of a "cable plant" specification can be found here
Department of Homeland Security's STRUCTURED CABLE PLANT STANDARD
A physical plant refers to the actual physical infrastructure that supports the operation of something. For example, a school needs buildings, and those buildings need heating and cooling and electricity, so they need boilers and air conditioners and electrical wiring, and the school needs roads so you can get to the buildings, and so forth, as you can see in this description from Farmingdale State College:
The Physical Plant at Farmingdale State College consists of: Power Plant, Structural Maintenance, Mechanical Equipment Maintenance (including heating, utilities, and air conditioning), maintenance of roads and grounds, parking lots, eight miles of primary and secondary roads, transportation, moving and trucking, utilities distribution (electric, gas, and domestic hot and cold water, steam and hot water for heating), sewage treatment, capital equipment, facilities planning, capital improvement budget, construction management, campus safety, custodial service, energy management and central receiving.
(source: https://www.farmingdale.edu/administration/administration-finance/physical-plant/)
So in this case, physical cabling plant probably refers to the physical installation of cables in the network, because the physical cables are necessary to actually do anything with the network, and security of the actual physical cables, racks, and networking hardware that you can touch is part of the overall network security plan.
