A "ground loop" is a out of control turn that you can't stop. The tail of the plane skids around and the inertia of the turn causes the plane to tip resulting in one wingtip dragging the runway. The centrifugal force of the turn acting on the plane's CG tries to make the plane turn even more and overwhelms the traction of the tailwheel causing it to skid. It's most likely to happen during high speed taxiing down wind or during a downwind landing. When taxiing upwind or landing into the wind, the airspeed is high enough for the rudder to be effective.