Here's a great summary of the different signal levels:
Mic level is the lowest, or weakest, level signal of the four and requires a preamplifier to bring it up to Line level.
Instrument level signals live between mic and line level signals and have the most variation. You typically see this kind of signal
come from an electric guitar or bass. A preamplifier is required to
bring the signal up to line level.
Line level signals are the highest level signals before amplification. This is the type of signal that typically flows through
your recording system after the preamplifier stage and before the
amplifier that powers your speakers.
Speaker level signals are post amplification. After a line level signal enters an amplifier, it exists to the speakers at speaker
level. These signals are much higher in voltage than line level and
require speaker cables for safe signal transfer.
Highly recommend reading the source as it has some more info.
Regarding balanced vs unbalanced:
Balanced audio cables help protect against electromagnetic interference. Electromagnetic interference can deterioate an audio signal and introduce noise when an audio signal travels over a long, unbalanced cable.
Udemy has a free "Audio Engineering 101" course that teaches signal flow, this would definitely be a great place to start, especially as it's only 30 minutes long.
Hope this helped!