High Level and Lowevel Signal?

high-level usually means a signal that has already been amplified.  For example, the wires on a radio's harness that go to each speaker could be consider high-level.  Low-level signals are those that come from a deck's preamp output (RCAs).  These signals have not yet been amplified until they reach an external amplifier.

When connecting external amplifiers it's generally best to use a head unit's low-level outputs to send the signal to the amp.  This provides the cleanest signal and lets the amplifier do the work of making the signal loud enough to power speakers.

low-level is usually a cleaner signal but it needs to be amplified in order to be of any value.   high-level is less clean because it's already been run through an amplifier, which by its very nature will alter the original signal.