A ghost note isn't really a note, it's a percussive thunk where you mute the string while striking it. No pitch, just rhythm. Funk bass lines are 30-50% ghost notes.
How to play a ghost note
Lightly rest the fingers of your fretting hand on the strings without pressing them down to the fret. Strike the string (with finger, pick, or thumb slap). The string can't vibrate (because you're muting it), so you hear a muted "thunk" instead of a pitched note.
In tab, ghost notes are marked as X: E|--X--X--3--X-- means "ghost, ghost, fret 3, ghost."
Why they matter
Pitched notes carry information (which chord, which key). Ghost notes carry rhythm. By interleaving them, you get a line that grooves but doesn't clutter the harmony.
Drill: ghost + slap
80 BPM. Play this on the E string:
beat: 1 2 3 4
note: E X E X
E = open E (real note), X = ghost note (muted thunk). Use your thumb to slap both, but mute the string with your fretting hand for X.
In a slap pattern
The classic slap pattern with ghosts:
beat: 1 & 2 & 3 & 4 &
play: T . P X T . P X
Slap, rest, pop, ghost, slap, rest, pop, ghost. The ghost note fills the space between the pop and the next slap. You can feel the groove tighten.
90 BPM. Try the slap-pop-ghost pattern. Loop on a single chord (E minor) for five minutes. Listen for the difference between the pitched notes and the ghost notes.
Where ghosts live in non-slap lines
Even in fingerstyle, ghost notes are useful. James Jamerson (Motown's session bassist) used them heavily in his fingerstyle lines. They're not exclusive to slap.
Next: funk grooves with 16th notes.