In Rayman Origins, imprisoned Electoons are trapped inside cages.
Cages are a central collectible and rescue objective in Rayman Origins. They appear throughout the Glade of Dreams, often at the end of a level or inside hidden passages. Players usually have to defeat the enemies guarding a cage before its protective force field disappears and Rayman can break it open.
The rescued Electoons are important to progression. They help rebuild connections between areas of the Glade and unlock routes, characters, challenges and other rewards. Their cries for help can also guide players toward secret sections that are easy to miss during a first run through a level.
Rayman Origins contains 108 cages according to detailed game references: 62 are hidden in secret chambers and 46 appear at level ends. Not every Electoon comes from a cage; others are earned through Lum collections and time trials. That distinction explains why chests, crystals and bottles can feel like plausible objects in the game without being the prisons described by this question.