Alternatively referred to as audio in and sound in, the line in or line-in is a jack found on computer sound cards that enables a user to connect an external audio device. These devices include CD players, audio mixers, musical instruments, and microphones. They are used to record, play, and modify the incoming audio.
The picture of a desktop sound card shows the blue line-in port represented as an arrow pointing into sound waves.
If you are using a laptop computer or other device that does not have a line in connection there are also many USB line-in devices. If your device also does not have a USB connection, your only form of sound input would be the internal microphone.