I've seen a few xx on different tabs and I was wondering. What does that mean? And also the x one too?
It means mute the note.
Usually just cover up the string specified to mute and do not press hard enough to sound the note, but so you get a click sound.
Also, make sure no harmonic tones are present when you mute.
they are a muted note. well not realy a note, its just a percussion noise.

lightly rest your fingers on the strings with your freting hand and and strike the said strings.
Please see the FAQ first if you come across any other symbols you don't recognise.
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