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.