In the image below I've shown some info about audio signals, the point being that the 2 green arrows going to the same cable show how 2 different types of audio signal can go through the same cable.
The confusing part is that you can use 1 cable for more than 1 purpose. Ie the lower cable in the above diagram can be used for a balanced mic cable (usually the case for XLR type connectors), but can also be used for stereo audio ( such in the case of headphone cables). The difference in each use can be seen in the bottom two waveforms in the above diagram.
The reason for using 'balanced' audio signals is for noise rejection on the length of the cable.
Below is a cable that converts from stereo 1/4" to two mono 1/4" cables, I made maybe a decade ago and fixed today. This cable can be used to plug a portable audio device with just a headphones socket into a mixer that doesnt have a stereo TRS socket (all mixers).
To clarify, if you plug a stereo signal into a socket that is expecting a balanced signal, the sound will be ruined, as explained in the waveforms.
Just to make this more complicated, if you use an unbalanced TRS plug in a balanced input socket, you will be fine, as the inverted signal will just be ground.
1/4" stereo socket to dual 1/4" mono, circa 10 yrs ago
Video on balanced cables:
A TRS connector (the one above is a 1/4" jack) can be used for balanced audio, as well as stereo audio, although the pinout is very different for each. For balanced, the Tip and Ring are the signal and the inverse signal. For stereo, the Tip is left channel and the Ring is right channel. In both cases the sleeve is ground.
In the case of unbalanced mono audio, there is only a tip (signal) and a sleeve (ground) required (Ring pin can be connected to Sleeve pin), this is the case with guitar leads (TS connector).Note: If you plug a stereo signal into a balanced socket, the 'differential amplifier' (described in the video above) will try to cancel the signals and you will get a really crappy audio signal. The socket is expecting balanced mono (figure above) but it's getting unbalanced stereo (figure above).
XLR / cannon:
XLR connectors can be used for balanced audio. In order to put unbalanced audio through an XLR connector, usually one of the signal pins is connected to ground, effectively the same as connecting the Ring in a TRS connector to the Sleeve above.
RCA connectors can be used for unbalanced mono audio. Inner is signal and outer is ground. Often used for Composite Video + Left and Right audio.