Line in what is that for - RCA Electronics - Others

In general:

Line In is the standard name for any generic audio input. Line Out would be for the corresponding output.

It allows a standard for audio equipment manufacturers to use when designing for interconnection.

http://en.wikipedia.org/wiki/Line_level

"Consumer electronic devices concerned with audio (for example Sound cards) often have a connector labeled "line in" and/or "line out". Line out provides an audio signal output and line in receives a signal input. The line in/out connections on a computer sound card are generally unbalanced, with a TRS connector of 6.35 mm (1/4"), 3.5 mm (1/8" miniature) or 2.5 mm (3/32" subminiature). The connections on most other consumer equipment use RCA jacks. In most cases changing the volume setting on the source equipment does not vary the strength of the line out signal."