The Anti-possession symbol will protect the wearer from possession by a demon.
Following Sam Winchester's possession by the demon Meg in 2.14 Born Under a Bad Sign, Bobby Singer gives Sam and Dean each a charm amulet with the symbol. Later in 3.12 Jus in Bello, the boys reveal that they have had the symbol tattooed on their chests. Other people with the anti-possession tattoo included Kevin Tran and his mother Linda Tran, Jack (though due to his being a Nephilim, the tattoos instantly healed) and Mary Winchester.
In 1.01 Pilot (The Winchesters), both John Winchester and Mary Campbell are in possession of the charm -- Mary wears one on a bracelet, while John's charm is connected to a key given to him that opens a Men of Letters Clubhouse.