"The dire creatures may be seen only by the damned or through an object scorched with holy fire."
Hellhound glasses are spectacles made of glass that has been scorched in holy fire, through which hellhounds are made clearly visible. Dean and Sam use them to fight and kill a hellhound that arrives to retrieve damned souls, 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 the trunk of the Impala, pours a little on the ground, lights it and scorches both glasses through the holy fire. While 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, Sam arrives just in time to notice this, and grabs the knife and cuts open the hellhound thus killing it.
When the hellhound Ramsey is released from Hell and begins attacking and killing people, Sam, Dean, and Crowley try to track her down. Sam and Dean retrieve the glasses from the trunk and while Dean and Crowley go through the woods looking for her, Sam stays behind with Gwen Hernandez.
Ramsey soon arrives at the Impala and jumps on top of the car. Sam retrieves an angel blade and attempts to confront her. Ramsey leaps at him and pins him to the ground, knocking off his glasses. Gwen scrambles in the back seat for a weapon and grabs the green cooler. She blindly clobbers Ramsey with it, giving Sam time to get to his feet, but he is still unable to see Ramsey without his glasses. He can see her footfalls raising dust and waits for her to jump again, then stabs her with the angel blade, killing her.