A gutter apron is an L-shaped flashing, similar to drip edge but with a longer leg, that bridges the gap between the roof edge and the back of the gutter so water can't run behind it.
On many homes the shingles overhang the fascia just enough that runoff can sneak behind the gutter and down the fascia board. A gutter apron tucks under the shingles and extends over the gutter's back lip, closing that gap and forcing all the water into the trough. It's an inexpensive add during installation or repair that prevents a common and damaging leak point. If you see staining on the fascia directly behind the gutter, a missing or short gutter apron is a likely culprit.
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