I have no idea what they are.
I can tell you, though, that I met a guy who saw one and told me about it in the late 1970's. That is: he told me about it in the late 1970's. This was before they became incorporated into UFO lore.
He said he and a friend were lying out in a field on a hot summer night looking up at the stars. A huge triangular shape passed slowly over them. They couldn't make out any details or features. The only reason they were aware of it is because it blocked out a huge triangular section of the sky as it went over.