Line in or line-in is a jack found on computer sound cards enabling users to connect an external audio device. These devices include CD players, audio mixers, musical instruments, and microphones. They're used to record, play, and modify the incoming audio.
Line in may be alternatively referred to as audio in and sound in.
The picture of a desktop sound card shows the blue line-in port represented as an arrow pointing into sound waves.
If you're using a laptop computer or other device that does not have a line-in connection, there are many USB line-in devices. If your device also does not have a USB connection, your only form of sound input is the internal microphone.
Using the line-in jack allows the computer to accept audio input. This input is often your voice that can be used as voice input using a microphone. However, this jack could also be used to record a musical instrument. To record the audio of whatever is connected to the line in port, you can use a sound recorder like Microsoft Sound Recorder, Audacity, or another sound recorder.