It means "Invisible Ancients" in Russian. 

We don't know anything about them, their habits, origin, physiological properties and the way they live. But we know that they are pretty old and ethtereal in their appereance, so why can't we call them using avalible info, it's simple but we can identify them from other species very easiely by this name. But i think it's somewhat lame to call them "Invisible Ancients" so in Russian it is. By the way, the true pronunciation is: [nʲɪzrʲˈimᵻjə drʲˈevnʲɪjə].