This is a sign used by stereo headphones to distinguish between left and Right channels, and R is the abbreviation of right, which stands for right channel; L is the abbreviation of Left, which stands for left channel.

Stereo refers to the stereo sound.

The sound emitted by nature is stereo, but if we amplify these stereo recordings and play them back, all the sounds are emitted from one speaker, and this reproduction (relative to the original sound source) is not stereo. At this time, because all kinds of sounds are emitted from the same speaker, the original sense of space (especially the sense of spatial distribution of sound groups) has disappeared. This playback is called mono.

If the whole system from recording to playback can restore the original sense of space to a certain extent (complete restoration is impossible), then this reproduction with a certain degree of spatial distribution characteristics such as azimuth layering is called stereo in sound technology.