The drip edge is the corbelled masonry that is near the top of the chimney. It is designed to shed water off the chimney. Many times the mason will not properly grout these areas and water can leak into the corbeled masonry brick holes and eventually leak down the chimney. The photo shows drip edges that have not been properly sealed and are leaking into the chimney. The mason should have layered mortar in these holes and filled this corbeled drip edge. When the water sits in these holes, it either travels down the chimney or sits and produces mold formation.