Superman is an immigrant from the stars who came from a now understandably extinct alien race. His origin is placed by a long story in his hometown planet known as Krypton. Jor-El is the biological father of Kal-El(superman) who was a scientist to the kryptonian council. He recently discovered that Krypton was facing their doom. The kyryptonian people have made a living computer who's name was braniac, this computer can instantly act as if it were a living being. When I mean living being I mean it can act exactly like if it was to have a will of its own. it can also be honest or not. The doomsday that was arriving to krypton was true on the other hand it would seem braniac was lying. Jor-El knew the planet was in danger so Jor-El built a space ship for his son in order for him to escape. Right now you are asking " why didn't superman's family escape together?", its because that there was no time making a ship that big to get one family so he's son was their only option.