From Wikipedia; edited and amended by SP

Line level is a term used to denote the strength of an audio signal used to transmit analog sound information between audio components such as CD and DVD players, TVs, audio amplifiers, and mixing consoles, and sometimes MP3 players.
In contrast to line level, there are weaker audio signals, such as those from microphones and instrument pickups, and stronger signals, such as those used to drive headphones and loudspeakers.
The strength of the various signals does not necessarily correlate with the output voltage of a device; it also depends on the source's output impedance, or the amount of current available to drive different loads.

Line levels and their nominal voltage levels.
Use Nominal level Nominal level, VRMS
US professional line-level audio - balanced +4 dBu ~1.3
Consumer line-level audio - unbalanced −10 OR -20 dBV ~0.3

Line level in traditional signal paths
Acoustic sounds (such as voices or musical instruments) are often recorded with transducers (microphones and pickups) that produce weak electrical signals.
These signals must be amplified to line level, where they are more easily manipulated by other devices such as mixing consoles and tape recorders.
Such amplification is performed by a device known as a preamplifier or "preamp".
After manipulation at line level, signals are then typically sent to a device known as a power amplifier, where they are amplified to levels that can drive headphones or loudspeakers, which convert the signals back into sounds that can be heard through the air.
Most phonographs also have a low output level and require a preamp; typically, a home stereo amplifier will have a special phono input with a built-in preamp, which is much more sensitive than a standard, line-level input. The built-in phono preamp also accounts for the RIAA equalization curve resident to phonograph recordings.