A terminal server is a server that is used to help serial devices access local area networks or similar networks. Typically, this type of server does not provide many security features or otherwise restrict access. Terminal servers are often seen as simple connecting devices for integrating hardware components into a greater architecture.
Techopedia explains Terminal Server
Terminal servers can be set up in several different ways. They may connect through a network interface card (NIC) to an Ethernet or token ring LAN, or through a modem to a wide area network. Other kinds of terminal servers provide links through other hardware/software architectures. Some can be customized to do more than one kind of connection. Terminal servers can also use various protocols including TCP/IP.
Terminal servers can work effectively to link up dozens and dozens of pieces of hardware, including personal computers, printers and other hardware components. They act as centralized connectors in place of other more sophisticated network setups that provide more security for data in transit.