Q. What is a breeder reactor?
92U238 and 90Th232 aren't fissile materials but are abundant in nature. In the reactor these are able to be converted into a fissile material 94Pu239 and 92U233 respectively by absorption of neutrons.
The process of producing additionally fissile material in a reactor in this manner than consumed during the operation of the reactor is called breeding. A fast reactor is able to be designed to serve as a good breeder reactor.