Thorium is an alternate fuel that can be used in breeder reactors with fast or slow neutrons. It's particularly well-suited for thermal breeders, which work best in fluid fuel systems. Fluid fuel systems (like the now internet-famed Molten Salt Reactor) have lots of great capabilities that fall into the broader category of Gen-IV nuclear reactors (near infinite sustainability, super-safe, reduced waste, higher temperature, etc.).

There was some internet hype for a while that Thorium was a end-all/be-all but those were slightly misguided in the sense that all Gen-IV reactors are end-all/be-alls. The Thorium itself had very little to do with it. It was an attempt at rebranding nuclear. As a nuclear engineer I prefer precision and promote all Gen-IV stuff including Th-fueled ones. Maybe I should have gone with the rebrand.