A Hallow is a source of Mana that is trapped in the Fallen World. Hallows frequently possess a strong enough flow of magic that even Sleepers can sense the power of the areas. They often have a strong allure for both Awakened and Sleeper, although the latter don't really understand why. More importantly (from a mage's perspective) Sleepers are more likely to Awaken in these sacred places. Not all spots that evoke powerful sensations are currently Hallows, although many are, and many more have been in the past and have the potential to be again if they are properly reopened.
The strongest Hallows tend to be situated in high places, such as on mountaintops or the tips of towers. Other Hallows can occur anywhere, even in the dark between two clefts, or down a deep pit. There is no predicting where they will occur, although mages know that high places often host powerful ones, and so tend to search there.