Can you give me a very basic definition of Harmonic? Is it simply things we can do to the settings to enhance the frequency delivery?

Harmonics are different frequencies that are related to each other. Using equipment that is limited in frequency range, we can use harmonics to reach much higher frequencies. Also harmonics can be used to target many frequencies at the same time.

This is a good site to teach harmonics:
http://www.physicsclassroom.com/Class/sound/u11l2a.cfm