A wire frame is dipped in bubble solution and placed in front of a speaker. At certain frequencies, the soap film oscillates in symmetrical patterns called normal modes. A light is reflected off the soap film through a lens onto the wall to show these normal modes as the frequency of the function generator is increased.  The first set of normal modes occur at frequencies lower than we can hear with our ears, which is why you can’t hear the speaker in the video until the frequency is increased.