To find out what is happening here, first set the group velocity equal to one using the slider bar. You will now see a frozen pattern moving steadily to the right on the screen. This represents the amplitude of, say, a sound wave propagating to the right. Now increase the frequency using the lower slider bar. You will see the peaks crowding closer together. But why are they coming in these bunches? The applet is actually plotting the wave generated by two pure notes which are very close together, so beats are generated, a waa waa waa sound, each waa corresponding to one bunch of peaks, or one wavepacket as we shall call it.