I think the GP was using '...' as a standard "stuff removed for brevity" ellipsis.
However, since you asked:
In a YAML document, there is an implicit top-level object, either a hash/dict/associative array or a list/array depending on your formatting.
The '...' separates streams, or top-level objects, within a single file or IO stream.
YAML parsers generally stop when they hit the separator, even in languages that can do multiple assignment. If your IO stream behaves like a file handle, you can read it repeatedly into different variables until EOF.
Multiple streams in a YAML document are fairly uncommon. I think most people don't know they exist, but I appreciate the flexibility and use it whenever it makes sense.
So is this the equivalent of multiple JSON documents? I'm looking at:
http://yaml.org/spec/1.1/#id857577 and http://yaml.org/spec/1.1/#stream/information%20model
I'm actually having a hard time, even with some google-fu, finding information about how this might be used. YAML->json parsers have failed to handle docs with a '...' separating parts of a document so far. Would this triple-dot structure be used to create multiple JS objects? How would I find more information about how this might be used in practice?
I have a feeling this would be easier to figure out if I just went and played around with it in practice, but it also seems like something that could be documented in more beginner-friendly terms.
I'll probably be kicking myself once I figure it out, but I also only have a cursory understanding of how streams work. Maybe this is an opportunity to fix that. Once I figure it out, I'll respond to this if nobody else has.