Our present understanding both theoretically and experimentally of the subatomic world is based on ideas about symmetry. Symmetry plays an important role in all theories in particle physics. In the context of the so called Standard Model, we are able to describe the observed particles in terms of just six quarks and six leptons. These fundamental particles have intrinsic spin of one half. Their chirality (=handedness, from the Greek chira=hand) describes whether their spin is directed along or against their direction of motion. We can have a (classical) picture for a right-handed particle if we imagine it spinning about its direction of motion in the same way as one turns a standard corkscrew.
Quantum Chromodynamics (QCD) is the modern theory that describes the strong interaction between quarks. If we ignore the masses of the two lightest quarks u and d then the theory enjoys a chiral symmetry. In this case there is no coupling between left- and right-handed quarks in QCD and the theory remains invariant under the so-called chiral symmetry transformations. However, the observed hadron and meson masses are well explained by models in which the quark are massive. We have to accept the quark masses and we say that the chiral symmetry of QCD is broken (or hidden).