Mic level vs line level
Keeping this short and sweet to eliminate confusion.
Mic Level = Small voltages from microphones Usually around -60dBu > -30dBu You want this low so your microphone signal is not corrupted by noise because they are tiny signals.
Line level = bigger voltages from other audio equipment such as pre amps or to speakers especially over a longer distance RCA connectors = -10dBV (-7.7dBu) XLR = +4dBu (0 on a VU meter) You want this because its industry standard for lots of different equipment to keep the signal flow effective.
So To Conclude mic level vs line level:
Mic = For Microphones Line = For Linking Audio Devices Together (Speakers, Desks etc..)
Microphones measure tiny differences in voltage and so need to be amplified using gain. Where as Line level has been established as an industry standard signal for other audio devices to transmit signals without the need for amplification. Line level is a much stronger signal and so if you plug-in a microphone into line level it will distort because the signal hasn't been amplified properly.
Remember if you are going into a camera from a mixer to set the inputs to LINE level and set your reference tone to align the meters on your mixer and the camera so you have no problems. If going wireless via radio microphone packs then use MIC level.