Regeneration is the ability of an organism to give rise to a new organism or individual from their body parts.
If the body of Hydra is cut into pieces, then each body piece of Hydra can grow into a complete Hydra i.e., on cutting into pieces, the cells of the cut parts divide rapidly to make 'ball of cells'. The cells present in the 'ball of cells' move to their proper places within the ball where they have to form various organs and body parts of the organisms.