Ok, so how do you go about muting that D string. X means mute, right?

It's part of the verse riff from a Papa Roach song
Instead of arching your fingers to get the sound of the open strings, just simply arch less, so your index mutes the string and it doesn't make any sound.

And yes, X means mute.
its called octave chords. throw your index onto the A and your ring finger onto the G and you lay your index down onto the D string to mute it