I'm revising for my introductory particle physics exam and I've noticed SU(3) pop up a few times, it was never really explained and I don't really understand what it means. For example when talking about the conservation law of Colour, it says the symmetry is phase invariance under SU(3) group of QCD. What does that mean? I don't think I'll be rigorously examined on this but I'd like to be able to have a basic understanding of what it means and the ramifications of it. So if anyone could explain it and why that means for me at the moment in particle physics, that would be tremendously helpful!