SNO+ is a multi-purpose particle physics experiment studying all things neutrino. Neutrinos are very light elementary particles. They come from the Sun, their antiparticles (antineutrinos) come from nuclear reactors, and these two things (neutrinos and antineutrinos) might in fact be the same thing.
It sounds like we know shamefully little about neutrinos, which is more or less true. Hence SNO+, which is studying all of the above to figure this stuff out.