It means one of a few possibles : dimension transfer/meeting of some kind, and/or observation. The notion of being always watched, information gathering and such are alluded to often in the show. I suspect we'll be clued in fully eventually, as to which of these it is, perhaps even its both and so has different meanings depending on context. Sometimes its clearly dimensional, othertimes it does look more like observation...So, as to what it means, we know roughly, but not for sure. A blue flash does occur on the other side when a portal is opened, as mentioned in over there part one, so theres our first real solid clue..