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CP violation is the failure of a theory to be invariant under the combination of charge conjugation and parity. Under CPT invariance it is equivalent to time-reversal violation.
CP violation matters experimentally because a symmetry predicts a forbidden transition, not merely a small correction to a permitted one. The Christenson-Cronin-Fitch-Turlay kaon CP violation experiment found evidence for precisely such a transition: a long-lived neutral kaon decaying into two pions. Understanding that inference requires both two-pion kaon event reconstruction and the distinction between kaon CP eigenstates and the physical states of neutral kaon mixing.
CP violation refers to the phenomenon where the combined operations of charge conjugation (C) and parity (P) do not yield the same physics for certain processes. Charge conjugation transforms a particle into its antiparticle, while parity transformation involves flipping the spatial coordinates (like mirroring). In essence, if a physical process behaves differently when particles are swapped for antiparticles (C transformation) and mirrored (P transformation), then CP violation is occurring.