meaning of having harmonic on special signal
Perform an Internet search using these keywords:

fourier transform harmonics


For a specific example, try a search using these keywords:

square wave fourier transform harmonics


You can find some useful videos on YouTube that demonstrate the concepts of Fourier transforms, fundamentals, harmonics, etc. For example:

Fourier Series Animation (Square Wave)

(^^- This is a visual example that adds odd harmonics (n=1,3,5,7) to build a square wave signal.)

Sine Wave to Square Wave Using Fourier Series

(^^- This is an auditory example. It's a bit loud, so be forewarned!)