The thorium fuel cycle is the path that thorium transmutes through from fertile source fuel to uranium fuel ready for fission. Th-232 absorbs a neutron, transmuting it into Th-233. Th-233 beta decays to Pa-233 and finally undergoes a second beta minus decay to become U-233. This is the one way of turning natural and abundant Th-232 into something fissionable. Since U-233 is not naturally found but makes an ideal nuclear reactor fuel, it is a much sought-after fuel cycle. This fuel cycle is of extreme importance to molten salt reactors, as most of the proposed reactors could use the thorium fuel cycle.
The thorium fuel cycle is important in the context of nuclear fission, as it is one of the ways that could supply nuclear reactors with fuel, although as of this writing it has not yet been done commercially. Th-232 can be used as a fertile fuel in many reactor types, including CANDU reactors and many types of molten salt reactors. A fertile fuel is a fuel that must absorb a neutron in order to become a fissile fuel. Fissile fuels are nuclear fuels that upon the absorption of a neutron undergo fission and release energy. In this situation the fissile fuel, which is the fuel powering the reactor, is U-233.