In Star Trek II: The Wrath of Khan, the Genesis Device rapidly transforms a dead planet into a living world.
The device was developed by Dr. Carol Marcus and her research team as a revolutionary terraforming technology. It uses a Genesis torpedo to deliver the effect, restructuring matter and creating an environment capable of supporting life.
The technology is central to the film's conflict because Starfleet considers Genesis a scientific breakthrough, while others see it as a potential weapon. Khan Noonien Singh seeks the device after learning of its power.
A common mix-up is calling the torpedo itself the Genesis Device. The torpedo is the delivery mechanism; Genesis is the broader technology and project. The later film Star Trek III: The Search for Spock shows the consequences of using Genesis on a planetary body.