Cain's burial site is a location where victims of Cain are buried.
After Cain fell back under the influence of the Mark of Cain, he decided that his bloodline was "tainted" and needed to be destroyed, since it contained many criminals and murderers, even the nicer ones could not be spared.
Cain began hunting down his own descendants at some point in 2015. It wasn't until he kidnapped and murdered serial killer Tommy Tolliver that he drew in the attention of Sam and Dean Winchester, as well as the angel Castiel, who discovered this location.
Castiel noted that among the buried were women and children. There was even a stuffed teddy bear upon one of the graves. The location was made out of a forest, with all its trees cut down.
Had Dean Winchester not managed to defeat Cain, then many more, including himself, would have been buried here.