When you are stunned, you phase out of existence temporarily causing all melee, ranged and spell attacks to miss you for 2 sec. This effect has a 10 sec cooldown.
Dematerialize is a passive monk ability learned at level 45 for those with the Mistweaver specialization. While active, melee, ranged, and spell attacks will miss the monk for a short period of time. Note the effect has an internal 10 second cooldown.