The BNC T is required to connect a BNC Terminator to the end of a cable that connects to a BNC input. By providing the necessary termination, degradation of the signal carried by the BNC cable is minimized. Like all BNC connectors, the BNC Terminator has characteristic impedance that matches the cable and terminator. For connections use in digital audio this impedance is 75 Ohms. The other common impedance for BNC connectors is 50 Ohms. For best performance, it is important that the BNC Terminator be of 75 Ohm impedance.
Please see termination more information.