A harmonic is a chime like sound created by lightly touching a vibrating string with the left hand at certain points along the fretboard. The left hand must be positioned directly above the fret just touching the string (i.e. without pressing down).
Harmonics are most easily played on the 12th, 7th and 5th frets. In the example below harmonics are played on the 5th fret of each string.
Harmonics are also useful for tuning the bass guitar (see appendix on tuning). To get a brighter sound, which is essential when playing harmonics, position the right hand closer to the bridge (back pick-up). In Lick 32 all notes are played at the 5th fret by barring the first finger across all 4 strings (see photo).
In Lick 34 all the notes are played on the 7th fret.
Lick 35 starts with a pick-up note and uses harmonics at the 7th fret on all 4 strings.