What does line-in mean on the sound control application?

I am not sure which channels to turn on on the volume control device. There are volume, microphone, line-in and CD to choose from for og devices (Oss Mixer?). Alsa PCM Master Capture, Alsa Capture Monitor source and Alsa Playback plus VIA Alsa Mixer Playback which includes mic, line-in, PCM and CD.