It's helium-3, which has two protons and one neutron. Neutrons are vital to fission reactions, but it's troublesome to deal with in fusion. Neutron radiation can be absorbed by atomic nuclei, changing their isotope, forcing electrons off and potentially even making them radioactive in the process. Good for your fission pile, not good for your fusion reactor, and very difficult to block.

The easiest form of fusion, deuterium-tritium fusion, creates a free neutron in the process. Deuterium-deuterium fusion does the same, though not at the same rate.

He-3 is a potential fusion fuel that doesn't create neutron radiation when you fuse it with more He-3, because it fuses into the stable He-4 (two protons and two neutrons) and two free protons.