A 50 ohm system means that the ratio V/I = 50. This is an impedance that is used for coaxial cables (for domestic use or video, 75 ohms is used). So in an amplifier the transistor might have an output impedance of say 100 ohms. A matching network would then be needed to reduce the impedance to 50. As every one uses 50, you can buy 50 ohm system components from any manufacturer.
Frank