A katana is a Japanese samurai sword.
While researching Shojo, 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 Shojo is at the local brewery, Dean heads there and attempts to fight it with the sword. However, as Dean is not drunk, the Shojo 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 Shojo, directed by Sam who is able to see the monster. Finally, despite his disadvantage of not being able to see the Shojo, 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 Shojo, 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 Shojo was trying to pull the katana to itself and Dean was in the way.