Get an acoustic, preferably a nylon, and with no other fingers touching the string, tap one of the high strings at a high fret, say 17 for example. You may notice that you hear two pitches. Why? Because what is going on is you have made the string vibrate between the bridge and the fret, but also behind your finger; between the fret behind your finger and the nut.
Now try this. On the high 'E' string, fret the note at the 9th fret and pick the string with your right hand, behind the left, I. e: between the fretted note and the nut. You will notice a high, harplike pitch, hence the name 'harping'. Experiment with different fret positions, different strings, different types of string, different types of guitars, effects, techniques, and so on, and you can get some cool effects.