Dragonglass is a very powerful tool when you are battling the undead. It allows you to vanquish Whitewalkers. By vanquishing the Whitewalker, all of his wights are vanquished too.
It is a magical volcano glass; mined mostly in Dragonstone, that can be fashioned into arrowheads and used with a bow or on placed on top of spears.
Keep a piece under your pillow in case a Whitewalker should visit in your sleep.
arrowhead, about 1" long, comes with card explaining its properties