It's an input that receives audio. On a TV this would be your red and white RCA cables. On a mixing console for recording music it could be an XLR or 1/4" cable. For MP3 Players it can be a 1/8" cable. That's the basics of it. There is a lot more options for audio cables and inputs, those are just the main ones.
Simply put, it is an "extra" audio input that allows a device to accept the audio signal from an external audio player.
Audio input is bringing sound signals into a device from another device, typically using a plug or jack.
If I understand the question, you are trying to intertwine different functions. Like your ears, the audio input jack receives an audio source, while the audio output jack transmits outputs audio content - similar to your mouth. If you are trying to redirect the input audio directly to the audio output port you will need a "passthru" (aka monitor) function. -P2E
Audio in is and audio input that can be used for recording purposes http://www.audio-dvd-maker.com
That would be where you connect anything with a STEREO AUDIO OUTPUT.
Audio: Line-in; Microphone
yes
Audio adapters are used for converting a plug into another style of plug. Audio adapters simply help one change the input plug of a certain system to a different input plug.
no
turntable
The most common used audio input device is the webcam
If your home theater receiver has an optical audio input, plug it from the output of the TV to the input of the receiver.