Why are some episodes marked as "Specials"?

A special episode is an episode which is linked to a series but is not necessarily part of the story and therefore not necessary to watch. It is however sometimes interesting to better understand the end of a season, etc.

Special episodes on BetaSeries are preceded by the "S" icon; you can be at 100% on a show by watching them or hiding them (using the "I don't want to watch this episode" option).
Just like other episodes, you can hide special episodes that you don't want to watch from your watchlist. If you don't want to see them at all, you can turn them off in the app settings (cogwheel on the top left of your profile).