Flashing is a strip of aluminum that is installed for a couple of different reasons. It can be used to bridge the gap between the metal drip edge under the first row shingles and the top of the guttering

Flashing is also installed to prevent water from running behind your new gutters due to shingles that may be curled down. (Curled down shingles can be prevented if the roofing company uses a drip edge, which we highly recommend. Do not allow a roofing company to leave the shingles extra long and let them fold down into the gutter.) It’s surprising how many roofing companies think this is acceptable