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This is a picture of the ceiling inside one of the vaulted chambers inside Snefru's Red Pyramid.  The ceiling is called a "corbelled" ceiling.  That means that each layer of stone overlaps the one beneath it by a little bit on both sides until the two sides meat at the top.  This is one of the very first archways built in stone (Snefru did the same thing inside his Bent Pyramid).  What makes this important is it relieves the stress created by all of the weight of all of the stone above the chamber and keeps the pyramid from crushing the room inside it.  The Pharaoh Khufu used this corbelled ceiling to create the Grand Gallery inside his own Pyramid.  The Grand Gallery inside the Great Pyramid of Khufu is one of the most remarkable interior spaces ever built.