I'm unconvinced about the amount of gold plating, but in my personal view the performance difference between gold plated and ordinary audio connectors is vastly overrated. Nor am I sure that they would stand up to repeated plugging and unplugging in, say, a mobile disc-jockey environment. But, as a fit and forget item for the price of a couple of mouthfuls of beer, they are unbeatable.
Although the RCA plug/socket started life as an audio connector, some equipment uses the standard for other signals as well, notably video. Colour codes are used to identify the sockets; typically red and white for stereo audio and yellow for video. I risk stating the obvious by saying that you would connect the black plug into the white socket.