Hollow Knight is a side-scrolling Metroidvania RPG that mainly focuses on exploration. As you play, your character will gain skills and abilities that will help you discover new areas and new items -- which, in turn, will help you open new areas and unlock new items. It's classic Metroidvania in a perfectly hand-drawn aesthetic.
And while the art of the characters in Hollow Knight appears to be quite simple, the background contrasts with it in such a way that makes the game to look incredible. That perfect juxtaposition of simple and complex set it apart from other games in the genre.