There are a total of 27 level transitions in Injustice: Gods Among Us. They are scattered across the 15 playable arenas. Ferris Aircraft and Atlantis are the only arenas without transitions.
The transitions basically allow you to move to a new area within an arena. New areas mean new interactive objects and will give you bonus XP after every successful fight.
Finding all transitions will also earn you the "Around the World" trophy or achievement.