Just a minor quibble, but the strictly correct phrase is resonance frequency, ie. frequency of the mode or resonance.
In this example the resonance in question is the fundamental resonance of the cavity formed by the glass. Forcing the system at the same frequency as the resonance gives the greatest response. Once the amplitude response exceeds the structural limits the glass will crack or possibly smash, although an unamplified human voice would likely require a very thin glass with low damping and no imperfections - hardly something you’d like to smash or, indeed, drink from!