There's a particular particle, the kaon, which can be created. This particle is highly unstable, and so, decays rapidly into other particles. Ever so often, it doesn't decay down the normal route but instead decays into a pion. This is extremely rare (6 in a billion).

In physics, we have what's called the "standard model". It's our best guess as to how physics works at the fundamental level. It's incomplete, however, with multiple slight variations. This decay pathway is interesting because it is quite sensitive to differences between these models. By measuring the energy and ratio of the resulting mess, we can disguard some variants of the model (their predicted energy is too high or too low).

By using a large number of little measurements, like this, scientists can home in on the most accurate "standard model" variant. This, in turn, informs work on a deeper understanding of physics.

Basically, a decade's work to put a single new point onto a graph. A point that only theoretical physicists care about, and might, or might not be useful down the line. Welcome to modern physics.