Drip edge is a perimeter flashing metal that protects the fascia and prevents wind blown rain from getting under the shingles. As water runs off of the roof, the process known as capillary action causes the water to adhere to the shingle and wick underneath to the plywood sheathing and fascia board. You can experiment with capillary action by attempting to pour water out of a glass. You will notice that water tends to hug the side of the glass rather than pour away from it. Drip edge is an inexpensive way to protect from expensive repairs down the road.