I would express it differently. At least this is what I used to do with harmonic analysis. I imagine putting my object on a shaking table (or I have a vibrating thing, like a motor, somewhere in the model). I want to know how the system answers (amplitude and phase at each node) to the different frequencies. But the best you can do is to read accurately the description in the very useful "structural analysis manual". It is in the APDL section but for me it remains the best :-)
So please read the description in: Help > Mechanical APDL > Structural Analysis Guide > Harmonic Analysis > 4.1 Uses for Harmonic Analysis
I copy a part of it that in my opinion confirms my point of view: "Harmonic analysis is a technique used to determine the steady-state response of a linear structure to loads that vary sinusoidally (harmonically) with time. The idea is to calculate the structure's response at several frequencies and obtain a graph of some response quantity (usually displacements) versus frequency. "Peak" responses are then identified on the graph and stresses reviewed at those peak frequencies."