Parity involves a transformation that changes the algebraic sign of the coordinate system.Parity is an important idea in quantum mechanics because the wavefunctions which represent particles can behave in different ways upon transformation of the coordinate system which describes them. Under the parity transformation:
The parity transformation changes a right-handed coordinate system into a left-handed one or vice versa. Two applications of the parity transformation restores the coordinate system to its original state.
It is a reasonable presupposition that nature should not care whether its coordinate system is right-handed or left-handed, but surprisingly, that turns out not to be so. In a famous experiment by C. S. Wu, the non-conservation of parity in beta decay was demonstrated.
Conservation laws for particles
Non-conservation of Parity
This and subsequent experiments have consistently shown that a neutrino always has its intrinsic angular momentum (spin) pointed in the direction opposite its velocity. It is called a left-handed particle as a result. Anti-neutrinos have their spins parallel to their velocity and are therefore right-handed particles. Therefore we say that the neutrino has an intrinsic parity.
The idea that nature at a very fundamental level can tell the difference between "left-handed" and "right-handed" systems is a radical one. It was thought for a time that the combination of the parity operation (=P) and "charge conjugation" (changing each particle into its antiparticle = C) was an inviolate conservation law (CP invariance). But the study of the Kaon decay in 1964 showed a violation of CP. If you add time reversal (=T) to the picture, then it appears that the combination of all three leaves the system indistinguishable from the original (CPT invariance).