Line level refers to the typical strength of the audio signal produced by professional audio gears like guitars, mixers, signal-processing equipment and other consumer audio gear. Hence; line level signal.

Mic level is the strength of a microphone signal. Mic level is generally and in most cases very much lower in signal strength than line level.

Line level Input is simply the port in the device where you plug in your line level gear.



Note:
*Plugging a mic into a line level input will usually produce audio that is too low for any good use.
*Trying to run a line level gear through a mic input will often create distortions and risk damage to the input.
*Please note that many modern devices have inputs and outputs that can be configured to accept either line level or mic level.