Kohanim
According to Jewish law, a kohen (Jewish priest) is not allowed to ritually defile himself by entering a cemetery. The construction of the Ohel and the pathway leading to it solves this problem according to the law. Fences separate the pathway from the other graves in the cemetery, enabling a kohen to pass through. At the Ohel itself, which is open to the sky to eliminate problems of tumas meis ("impurity from the dead"; see Tumah and taharah) in an enclosure, a low wall surrounds the graves. The kohen should stand approximately 30 inches (760 mm) away from this wall to maintain his ritual purity.
Read more about this topic: Ohel (Chabad)