Yav (Jav), Prav and Nav are three categories of the world in old Slavic religion.

Yav

Yav is the material world in which we are right now. Gods however are part of Yav as much. Yav is contained within Nav.

Nav

Nav is immaterial world, the world of dead.
Stars, which are souls of dead, as well as Svarga and Irij are parts of Nav. The belief in Nav remained to this day, though now not clearly defined.

Prav

Prav is the
law of Svarog that governs both Yav and Nav. In modern world pandane to this concept are laws of physics, but there is one big difference. The law that says that a stone falls when dropped is not only a mere observable fact: it is rightfull, good and just that stones are falling to the ground when dropped. Somewhat similar belief today is anthropic principle, but it only states that laws of physics are useful.

External link