When it comes to rhythm guitar, you need to learn how to stop the strings. Just listen to blues rhythm guitar and you'll hear that it’s not one repetitive wall of sound, but an open, varied sound with breathing room and subtle breaks.
These breaks are what prevent the chord strums from running together. The little gaps in sound keep the music sounding crisp and controlled. To create a rhythm guitar part with some breathing space between the notes, you need to momentarily stop the strings from ringing. You can stop the strings instantly with the left hand — letting the left hand go limp is the best and quickest way to stop a string from ringing — far faster and more controlled than anything you can do with the right hand.
To get the left hand to mute (indicated by an X note head), just slightly relax the fretting fingers enough to release pressure on the fretted strings. The strings instantly deaden, completely cutting off the sound. The muted strings intermixed with the sounding strings create a percussive and syncopated rhythm.
Meanwhile, if your right hand keeps going in the established strumming pattern, you produce a satisfying thunk sound as the right hand hits all these deadened strings. By allowing your left hand to mute, you can keep your right hand going uninterrupted, in alternating down- and upstrokes.
