XX's on tab
# 1
It means to mute them, some times it's in a spot where it's easier to let a fretting finger touch the string to mute it.
# 2
It might also mean to not play the string at all, by just avoiding it.
For instance a D chord might be notated like this:
------2--------
------3--------
------2--------
------0--------
------X--------
------X--------
You could either mute the 5th and 6th strings (which will make your playing a lot cleaner the sooner you start learning to do this) or just avoid playing them by only strumming strings 1 thru 4
For instance a D chord might be notated like this:
------2--------
------3--------
------2--------
------0--------
------X--------
------X--------
You could either mute the 5th and 6th strings (which will make your playing a lot cleaner the sooner you start learning to do this) or just avoid playing them by only strumming strings 1 thru 4
# 3