Its a medical solvent that the Awoken produce from a plant called Queensfoil. They put it's leaves through some sort of process and likely dissolve them in an alcohol, as the term "tincture" suggests. Tinctures of Queensfoil doesn't cause the drinker to Ascend in the same sense that the Hive do, rather only cause hallucinogenic visions that allow one to see past the fabric of reality and more easily slip into and out from other realities.