bad sci-fi jargon. I really wished they gave it a different name, something more symbolic or deep rather than flash gordenlike
as for what it is, from the game, it's a place where space and time have no meaning, nor does life. a person exists simultatniously every where and every when, but cannot occupy any specfic moment or place like one can in normal space-time. from what I can guess, the basic concept of matter we have here would also have no meaning there as its nature has become so fluid to allow infinite existence. This is my best guess to how Maggie and Brink can remerge from it alive with new bodies despite their old bodies having died and still existing in our space-time.
As for Brink's oldness, it may having something to do with how he came out of space-time 6, maybe at the wrong time or maybe it was a reflection of himself or how he felt about himself after looking at his selfish acts with the crystals, thus he chose a higher "probabilty of existence" or willingness to exist as someone old rather than his young self that he's ashamed of.