The Phantom Zone is where Krypton's criminals are sent.
Jor-El, in the hours before Krypton's destruction, argued that the Phantom Zone might be used to save the Kryptonian people — that they could be suspended within the Zone until a single individual might travel by starship to a habitable world, and then release the survivors. The Science Council roundly rejected the proposal, objecting to the idea of imprisoning innocent people alongside criminals.
Nonetheless, a Phantom Zone projector was sent to Earth with Kal-El when Jor-El used his prototype starship to save his son from Krypton's doom. Years later, Superman would inadvertently release the criminals Jax-Ur and Mala from the Zone. The two would wreak havoc on Earth, even imprisoning Superman himself in the Zone; Superman was soon freed with the assistance of Professor Emil Hamilton, and the two criminals were ultimately returned to their extradimensional prison.