Spelljammer is AD&D in fantasy space, just as Planescape is AD&D in the Outer Planes. Fantasy space is more... Ptolmaic? Aristotelian? than real life outer space, e.g. constellations are lights on the crystal shell around a solar system, not full-blown suns like our sun. It's full of urbane illithid info traders, beholder civil wars, elven biotechnology, superintelligent orcs called scro who recite insults in elvish prose, and planets covered in tarrasques. My 5E game is set against a SJ backdrop although the action often takes place on the players' home planet instead of wildspace--but they have been there and some NPCs are from there.