In the calendar today is a graph that is very important for the thorium fuel cycle...
Today I give you ETA!
ETA tells us about how many neutrons you get out for each neutron going in (to a nucleus) on average ("neutrons emitted/neutrons absorbed"). As you can see you get, on average, more neutrons from fission of uranium-233 (red line) than you get from plutonium-239 (blue line) - if the neutron going in has low energy/is a thermal neutron.
So, since uranium-233 is a better fissile material than plutonium-239 (when hit by low-energy neutrons), you can produce more new fissile material from thorium, than you can from uranium :D We like that <3<3<3