Ghost notes are notes achieved by dampening a string with your left hand while playing it. They have no distinguishable pitch, and are represented with an :. To dampen the string, lightly place your left hand fingers on it without pressing down.
Be careful not to produce any ringing sound (called a harmonic [see page 21] or to produce a hammered-on note).
For ease of playing, dampen the string in the same area of the fretboard as you are playing the other (fretted) notes.
It can be seen that each ghost note corresponds with one particular string.