To be fair, it is a little more elaborate than “it was all a dream,” but not much. As laid out by Metro, the idea some fans have latched onto is that all of Stranger Things has really been part of a big Dungeons & Dragons game and that none of it really happened. So, like, Will and Mike and Lucas and Dustin are sitting in the basement, rolling dice to determine stuff like “meet a girl with magic powers,” “Will gets trapped in a hell dimension,” and “lots of friends and family members die horribly.”
The evidence for this, presumably, is that the Stranger Things kids like to play Dungeons & Dragons and there are a handful of Dungeons & Dragons-inspired monsters and monster names in the series… which is pretty thin. Also, Dungeons & Dragons games tend to take place in a fantasy world with specific fantasy creatures and classes, not ‘80s America with a bunch of nerdy kids. So, in this theory, are we supposed to assume that these kids decided to play a game about themselves? And they included all of these other people they know? Or some other people created all of the Stranger Things characters and based a game around them? Because both of those are terrible.
Luckily, the Duffer Brothers shot the theory down, so Stranger Things will not end in such a stupid way. It may end in a totally different stupid way (they’re all living in a snow globe, they decide that Will gets to be king because he had the “best story,” Eleven ends up marrying Robin), buy not this stupid way.