The post-baking-video universe refers to any time after Dan posted their joint halloween baking monster pops video, or after October 31, 2016.
The phrase was coined by Dan himself during the third Impossible Quiz video (uploaded November 12, 2016), which was a very obvious reference to the baking video they uploaded less than two weeks prior.
On November 12, 2016, the same day they posted the Impossible Quiz video, Dan used the phrase in a twitter reply, thus immortalizing it as common phandom lingo.
But what does it mean?
The monster pops baking video rocked the phandom when it was uploaded, as the sheer amount of adult humour and nuanced behaviour in it was unlike anything they'd uploaded before. Referencing the post-baking universe essentially refers to any indication of open and relaxed change in Dan and Phil's actions with each other and their audience as a result.
The first time Dan brought up the post-baking universe was to address one of Phil's innuendos. However, it wasn't excused or dismissed as an accident like they often were in the past, and was merely treated as a result of being in the post-baking universe. Alongside that, the fact that Dan has acknowledged this shift in a liveshow (November 24, 2016) and a video (December 12, 2016), proves that it isn't just something the viewers picked up from the video and ran with, but rather something they view in a similar way on their end.
It seemed to mark an overall turning point in Dan and Phil's demeanours online. Since that point, they have been increasingly more natural and open, straying away from their more exaggerated personas.