Science fiction convention used to circumvent the fact that, according to conventional science, FTL space travel is probably impossible. A generation ship is a vast space ship that travels at sub-light speeds and is capable of sustaining human life for centuries or more. The idea is that if it takes generations to reach another star system then you do just that: the crew live their lives aboard the ship, breed, and then their children take control of the ship and then their children and so on until the generation ship actually arrives at its destination.