A katana is an ancient Japanese sword, most commonly used by the samurai of Feudal Japan.
While researching Shōjō, Sam learns that the only way to kill one is with a katana that has a Shinto blessing performed over it. While he and Garth work to protect the Shojo's next targets, Dean visits a pawn shop and buys a katana. He is then able to pay a Japanese chef to perform a Shinto blessing over it.
After learning that the Shōjō is at the local brewery, Dean heads there and attempts to fight it with the sword. However, as Dean is not drunk, the Shōjō is invisible to him and it knocks the sword from his hand. The ghost of Bobby Singer returns the sword to Dean who continues to attempt to kill the Shōjō, directed by Sam who is able to see the monster. Finally, despite his disadvantage of not being able to see the Shōjō, Dean stabs the katana through its chest, killing it. The katana then falls to the floor and is not seen again.
Following the battle with the Shōjō, the Winchesters discuss the idea that Bobby's ghost could've returned the katana to Dean. However, as they have seen no proof that Bobby's ghost is there, they dismiss the idea and suggest instead that the Shōjō was trying to pull the katana to itself and Dean was in the way.