Elf on the Shelf is a Christmas tradition where a small elf is placed in a household to watch over children and report their behavior back to Santa Claus. The elf is typically hidden in a secret location, and the children are told that the elf can move around when no one is looking. In reality, the elf is moved by an adult each night to a new location. The movement of the elf on the shelf is a fun and engaging way to build excitement and anticipation for Christmas.