Short Answer
Drip edge is metal flashing installed along roof edges to help direct water away from vulnerable roof-edge components. It can also help direct runoff into gutters.
Detailed Explanation
Drip edge is an L-shaped piece of metal flashing installed along the edges of the roof — both at the eaves (bottom edges) and the rakes (sloped edges). At the eaves, it directs water off the roof edge and into the gutter, preventing water from curling back under the roof deck and rotting the fascia. At the rakes, it protects the edge of the roofing from wind-driven rain. Drip edge also provides a clean, finished appearance at the roof edge and helps protect the edge of the underlayment from wind. It's a relatively inexpensive component, but its absence can lead to fascia rot, underlayment damage, and water running behind gutters.
Why It Matters
Drip edge is a small component with a big impact on protecting fascia and directing water into gutters. Its absence is a common corner-cutting measure.