Basically, Krypton is a very genetic advanced civilation, people don`t have sex or even get in contact with other kryptonians. They progressed so much over disease and all the problems that people live a lot of years. When a kryptonian dies, usuallly a male and a female are chosen to breed a kryptonian child without having never even met. They`re selected because they`re the best available. The infants are born into a gestation chamber. Lara and Jor-el are chosen to breed the new kryptonian child, Kal-el because people are dying over kryptonite exposure, called the plague, the radioactive fusion of elements in the instable core of Krypton.
But the thing is that Jor-el sees Lara and falls in love with her. So, once he discovers Krypton is going to explode, he take the fetus of Kal-el out of the gestation chamber, attaches a hyperdrive propulsor in it and launches into space, while saying i love you to lara.
It is much more complex than that, there`s a whole history with cloning and all that. It`s fascinating, IMO. I like it a lot because it feels very different from Earth.
Read World of Krypton written by John Byrne to know about all the details.