Complex sentence: A breeder reactor is a nuclear reactor capable of generating more fissile material than it consumes because its neutron economy is high enough to breed fissile fuel from fertile material like uranium-238 or thorium-232.
Split into simpler sentences: A breeder reactor is a nuclear reactor capable of generating more fissile material than it consumes. These devices are able to achieve this feat because their neutron economy is high enough to breed more fissile fuel they use from fertile material like uranium-238 or thorium-232.