Line level means audiosignals voltage what is typically mostly 2 Voltages peak to peak. It is common in CD-players and cassetteplayers. It is much more than microphonelevel which can be couple of milliVolts. Line level signal can be connect to amplifier which plays loudspeakers with much more power than linelevel uses because it's high currents.
Preamplifier is the device what can make linelevel signal example from microphone's weak signal.