Supernatural was created by Eric Kripke and premiered in 2005. It was initially planned for 3
expanded to 5, and then grew to fifteen. Over the course of the show,
while Sam and Dean stay as our anchoring protagonists, the cast and
world expanded into a sprawling complex universe.
Currently, Supernatural is on its fifteenth and final season with an impressive run of over 300 episodes. The show's longevity is thanks, in part, to its dedicated fan-base. A fan-base that the show itself acknowledges in-world, perhaps most famously in the season 10 episode: Fan Fiction.
The show has also had a large impact in pop-culture. Having been referenced in everything from comics, books and TV shows, to a tweet from the fast-food chain Arby's featuring an anti-possession symbol made out of ketchup.
Supernatural's blend of complex over arching stories and meta, self-referential humor leaves a lot of room for details that viewers might not catch in a first watch. With so many episodes, it's hard to blame you, so kudos to any true-blue fans who caught these all in a first viewing.