Flashings are used for water proofing. Usually installed in transition areas, flashings are used to direct water away from a particular area, and towards another.
Gutter flashings (a.k.a. gutter apron) are installed underneath the roofing and into the gutter. Gutter apron keeps the water from running behind the gutter. Flashings are 2" x 3" x 10 ft. long and are bent at a 35 degree angle.