Things that are fanned have the spread-out, winglike shape of an open fan. A peacock's fanned tail feathers are pretty spectacular to see.
You might browse through fanned scarves on a table at a craft fair, or choose one from the fanned playing cards that a magician offers you. In either case, the things that are fanned are spread out and shaped like a fan. The adjective fanned comes from the verb fan, which gained the meaning "spread like a hand-held fan" in the late 16th century, when hand-held fans were quite common.