Hellhound Glasses are spectacles made of glass that has been scorched in Holy Fire, through which hellhounds are clearly visible. Dean and Sam use them to fight and kill a hellhound that arrives to retrieve a damned soul, in order to complete the first of three trials to be able to shut the gates of Hell.
After being told by Kevin that hellhounds are only visible to damned souls or through glasses scorched with Holy Fire, Dean goes in search of some glasses at the farm's supply room. He finds a box full of them and takes a couple for Sam and himself. He later gathers Holy Oil from his Impala, pours a little on an open ground, lights it and scorches both glasses through the Holy Fire. He gives Sam a pair to use. Dean goes in search of the hellhound. After noticing where the hellhound is, he puts on the glasses and confronts it. Dean's glasses get knocked off him after being attacked by the hellhound, but Sam notices this, grabs the knife and cuts open the hellhound thus killing it.