Hexagon Levels are a feature used to mark levels that are hard. Fanon creators typically assign levels the Hexagon mark when they or fellow wiki users feel that a level is hard enough to warrant such a label.
In Candy Crush Saga: Candies Revenge, Hexagon levels are known as Hardcore Levels. Up to 6 levels in any given episode can be marked as Hardcore Levels.
Like in CCR, hexagon levels are known as Hardcore levels. Up to 5 levels in any given episode can be marked as Hardcore Levels.
See List of CCSS Hexagon levels to see all the hardcore levels.
In Courtemanche437's Candy Crush Saga, Hexagon levels are known as Challenge Levels. Like in CCR, up to six levels can be marked per episode.