Mic level is a very low voltage output level from a microphone when it captures sound and converts it into an electrical signal. It is the weakest output level possible and requires a preamp to bring it up to a line level signal. Mic level signals are typically not audible when monitored directly.
A Line level signal (+4 dBu or -10dBv) is more commonly used by stereo equipment, such as a CD/DVD player or the output level of an aux cord for a smartphone. Most ALL headphone output jacks provide a line level signal. Line level signals are audible and the audio is boosted by a preamp or amplifier of some sort.