what the hell are xxx, i.e ghost notes, and how do you play them?

You MUTE those with either your fretting hand(by laying just enough pressure on the neck to produce a "thud") or with your plucking hand(use your middle finger to hold/mute the string while your index finger plays the "note").
There's other methods(I'm sure)...

Try this-
Rhythm-/1.&.2.&.3.&.4.&./
String-/E.E.E.E.E.E.E.E./
Notes--/G.x.G.x.G.x.G.x./

On the downbeats, play the "G" note as usual; on the UPbeats, lift/lighten your fretting hand/finger enough to get the "X-notes".

Additionally: If the X is accompanied by a P or T, pop or slap the note respectively, with your right hand while muting the strings with your left. If the X is accompanied by a L or LH, give the fingerboard a slap with your left hand fingers.