the total harmonic distortion is the ratio between the signal power of
a) the fundamental signal frequency
b) the sum of all harmonic frequencies (2nd order, 3rd order, 4th order ,...)
It is a popular measurement to rate the amount of distortion in a signal.
The FX100 can measure THD at single frequencies as well as a response in a frequency or amplitude sweep.
I recommend this article to learn more about distortion: https://www.nti-audio.com/en/support/know-how/lets-clear-up-some-things-about-distortion
What is total harmonic distortion? How can we calculate THD with FX100?