“Mic level” is for low-voltage, unamplified microphone signals
Line level is a standard, relatively strong audio signal that requires minimal amplification compared to a weaker microphone-level signal.
The strong, “line-level” signal is already at a suitable voltage to allow the device to process, record, or play back the incoming audio. A “line-in” audio connection is a port or socket on a device, such as a computer, sound card, or audio mixer, that receives a high-level, amplified audio signal from an external source like a musical instrument, CD player, or pre-amplified microphone.
You can use a “headphone out” as a “line-out”, but you can’t use a “line-out” as a “headphone out” without amplification.