Anti-Possession Symbol
From Super-wiki
Following Sam's possession by Meg in 2.14 Born Under a Bad Sign, Bobby gives Sam and Dean each a charm amulet which features a symbol which will stop demonic possession. In 3.12 Jus in Bello, the boys reveal that they have had these symbols tattooed on their chests (as have many Supernatural fans). In 5.02 Good God, Y'All, Ellen reveals that Jo wears an anti-possession charm.