Traditionally, the Flex spot in the NFL includes RB, WR, or TE. Superflex allows for a QB to take this position.
While most contests are traditional Flex, Underdog will occasionally run a contest where a QB can be used in that spot, known as Superflex. Superflex contests will be clearly labeled as such, otherwise, the contest defaults to Flex.
For more information on NFL contests, please visit this link here.