Landroger - in brief:
In flight, the assorted rotors, engine(s) and drive shafts transmit their own vibrations to the structure which are generally stabilised because there is no real focal point for them to attach to.
On the ground, however, vibrations can focus through the landing gear - if its natural vibration matches that of the main rotor, every time a blade rotates, the vibrations can receive another reflected pulse to increase their amplitude, which could cause the aircraft to rock back and forth and eventually tip over and be destroyed. This is especially problematical if the C of G of the rotor disc is away from the mast* and creates a wobble, and if any damping effect from the undercarriage is not available (any oleos may be fully extended). Peculiar to helicopters with dragging hinges (which implies articulated heads), Ground Resonance is indicated by an uncontrollable lateral oscillation (because roll inertia is lower than for pitch) increasing rapidly in sympathy with rotor RPM.
*Hooke's Joint effect can cause this.
The official teaching (in the exams anyway) is to lift off if you have flying RPM and to cut the throttle if you haven't. Its best to do this quickly so that the dampers are all snapped back in line at once to stop any eccentric motion. One of Biggin Hill Helicopters' Schwiezers used to suffer from this a lot and they used to do an engine off landing from the hover every time.
Phil