I still don't understand relativity. But I've studied a lot of quantum physics. If someone asked me to define quantum physics, I'd say "very, very small things behave in REALLY WEIRD WAYS."
Einstein helped realize that light is comprised of tiny, massless, particles called photons. This video does a pretty good job explaining the particle-wave duality of electrons (and photons) and the uncertainty principle, some of basic principes of quantum theory.