There are a couple of fundamental symmetries of nature that, at least in everyday life, seem pretty obvious. One is that the laws of physics in a mirror -- where left and right are reversed -- are the same as our normal laws of physics. (We call that Parity, or P-symmetry.) Another is that matter and anti-matter obey the same laws of physics. (We call that Charge Conjugation, or C-symmetry.) Most laws of physics that you know, like gravity and electromagnetism, always obey these symmetries.
According to the standard model, they have to; it's coded into the physics. But these symmetries don't exist for the nuclear (weak and strong) forces in the standard model. If I took something like a muon, reflected it in the mirror (applying P-symmetry), and replaced that image with an anti-muon (applying C-symmetry), I'd be testing whether the combination of CP-symmetry was a good one or not.
If it were a good symmetry, then if all the muons decayed with one orientation, all the anti-muons would decay with that specific, mirrored orientation. But they don't, and so that CP-symmetry is violated. This is good for the Universe, because CP-violation is one of the necessary things to make more matter than anti-matter. But if it happens for an interaction like this -- the Weak nuclear interaction -- then it stands to reason that it should also happen for the strong nuclear force.
But it doesn't! Why wouldn't it?