The Connectronics B-75TM is a 75 ohm BNC terminator. The role of a BNC terminator is to absorb the electrical energy of the signal as it reaches the ends of a cable and avoids reflection of signals so it doesn't become noise. the BNC terminator is also used to close open terminations such as the a T junction where other extraneous signals can enter the circuit through that open point. Think of it like the gas cap on your car. If it wasn't there at the very least you would loose gas from the fumes leaking out. It also keeps foreign material from entering the tank which can muck up your engine. Same thing with a signal traveling through a BNC cable. If you are using a BNC T junction and one of the ends are "open" you can experience severe signal loss or you may experience signal noise from a foreign transmission.