If you really want to understand this drama and if you want the continuity of the Kingdom Series, I suggest you watch Kingdom Season 1 and Kingdom: Ashin of the North first and pay attention to every little detail of the series and the movie, respectively, if you want to be amazed with Kingdom season 2. Don’t get me wrong, it’s easy to miss out on a lot of details and yet still enjoy this season and the entire Kingdom series, but if you have that OC personality and you’re the type of person who cannot sleep, knowing you don’t understand a lot of things about the drama, you have to re-watch the Season 1 and watch the movie.
I hope you find my suggestion helpful and I hope your questions will be answered- like the origins of the resurrection plant, who discovered it, what really happens to the dead and how they become zombies after putting the plant on their glabella, do all people who get bitten by the “reanimated” become zombies, what can be done if a person gets bitten by the zombies.