Let us first define resonance. Resonance is a physical phenomena where it is observed that an object vibrates at higher amplitudes at certain frequencies . The amplitude of the vibrations at the resonant frequency is higher than that of the rest of the frequencies. This frequency is generally equal to the natural frequency of the object.
In the case of a 3D printer, when the printer is printing at speeds which are nearer to the resonant frequency, the printer would vibrate, which in turn can introduce ripples in the print. To test the printer for its behavior in such conditions, we use X-Y resonance and Z-resonance tests.
Resonance cannot be quantified and hence is always defined in binary, in terms of pass or fail values.