Line level is the strength of an audio signal. An audio signal transfers sound, for example to CD players and cassette players. It is much higher than microphone level, which can be only a couple of millivolts. A line level signal can be connected to an amplifier which sends the signal to loudspeakers with much higher power.
A preamplifier is a device that can make the line level higher. It can boost a microphone's weak audio signal.