In this case of the word "harmonic" appearing in the above transcription, it means that you lightly touch the strings above the indicated fret, right over the metal fret, with one hand, while struming the strings with your other hand. This creates ringing tones that are harmonics of the base tones of the strings. These are softer sounds, and different tones than what you would get if you would press down hard on the strings as you would for making a chord in the normal way. You will have to experiment. On some guitars, it is easier to make these tones than on others, but generally, you can make such tones most easily at the 5th, 7th, and 12th frets - it should be possible to get tones at these locations with the right technique. I think I also lift the hand over the fret away from the strings onces the tones have been activated by the other hand in order to prolong the ringing. Hope this helps.

What means "harmonic"?