Ember Tap is a Destructionwarlock ability, available at level 42. Ember Tap will spend a single Burning Ember to heal the Warlock.
While Ember Tap's standard healing is very weak, there are a number of effects that can significantly increase its healing potential. Which of these you should select will depend both on the type of content you are doing, whether you have healers in your party or are soloing, and how much healing said healers will be able to provide you with, if applicable.