guitar tabs question

[Stick up for ur mates]

I'm trying to play this song but I dont know what something means. Here is an example straight from the tab:

|-----------------------------------------------------------------------------|
|-----------------------------------------------------------------------------|
|--4----6----12---11---7----6-------------------------------------------------|
|--x----x----xx---xx---x----x-------------------------------------------------|
|--2----4----10---9----5----4-------------------------------------------------|
|-----------------------------------------------------------------------------|

What does the "x" mean in that tab?

 

[liver]

It means the string is muted. Rest your finger against the string so when it's plucked it doesn't ring out.