Line-in recording is a term often used by manufacturers of sound equipment to refer to the capability of a device to record line level

Line level

Line level is a term used to denote the strength of an audio signal used to transmit analog sound between audio components such as CD and DVD players, TVs, audio amplifiers, and mixing consoles, and sometimes MP3 players....

 audio feeds. Microphone

Microphone

A microphone is an acoustic-to-electric transducer or sensor that converts sound into an electrical signal. In 1877, Emile Berliner invented the first microphone used as a telephone voice transmitter...

and instrument inputs, by contrast, are designed for devices which require further amplification to be at line-level.

The common 3 mm line-in connector has the left channel on the tip and right channel in the middle the port is used to connect with other devices.