Quintessence: The building block of the soul and the life giving energy of the trees, Quintessence is all around our world and within all of us. It is generally broken down into three sub forms. Quintessence of the Mind, Body and Soul.  Harnessing control of one's own reserve of Quintessence is what allows for manipulation of the Threads of the Weave and the Quintessence throughout the world. Utilizing “points” of Quintessence is the way people unleash their innate magical abilities. For Some, this simply allows them to touch upon the threads of the world lightly. For others, this is how they unleash their Voice. Absorbing the light of the tree directly can refill one's Quintessence reserve but absorbing too much can break down your body and cause you to simply fade away into quintessence. 
As observed with the  Master's of the Rookery,  Consuming the fruit of the Great Trees seems to grant some  extra form of Quintessence. Or rather it  seems to give them an extra reserve of the energy. It is not fully understood as the number of people who have consumed the fruit are so small.