A Wickerman is a straw construct resembling a humanoid.
A Wickerman is burned by the Forsaken near the walls of Lordaeron during the Wickerman Festival to honor Queen Sylvanas and their escape from the Scourge. After the Cataclysm, Gilneas reintroduces the practice of burning the Wickerman to the Alliance. For the Alliance, the wickerman embodies any unrest, fear, or uncertainty within them. They set it ablaze with the hope of tempering themselves against whatever tomorrow may bring. Gilneans burn one outside of Stormwind City. Smaller wickermen are also burned in other parts of the world during Hallow's End in order to practice putting out fires from the attacks by the Headless Horseman. The Moonkin of Silvermyst Isle also have a Wickerman.
In the RPG
The Wicker Man was a large wicker statue of a human allegedly used by the ancient Druids (priests of Gallic paganism) for human sacrifice by burning it in effigy, according to Julius Caesar in his Commentarii de Bello Gallico (Commentary on the Gallic Wars). Wickermen are still used today in various neopagan and Wiccan groups but without the human sacrifice element.
It was also the name of a movie, The Wicker Man in 1973 and a remake also called The Wicker Man in 2006, which dealt with a form of Celtic paganism and the Wicker Man.