The animation shows three harmonics on a vibrating string. The top string is vibrating with the lowest frequency - this is the fundamental or first harmonic. The next is vibrating with a frequency double that of the first - this is the second harmonic and would give a note one octave above the fundamental. The lower string is vibrating four times faster than the top one - this is the fourth harmonic and would give a note two octaves above the first. (The third harmonic is not shown here).