Any train of waves that hits your break is basically made up of a combination of a large number of regular (constant amplitude & wavelength) waves
if you can imagine two regular wave trains of different wavelengths coinciding its easy enough to see that where peaks coincide the resulting wave will be at a maximum amplitude, where peak and trough they'll cancel out
If you can imagine a combination of a 100 such regular waves of different amlitude and wavelengths you can see how we end up with what is a seemingly random wavetrain
A 'set' wave is basically a larger peak than the majority of the other peaks in its vicinity.
That's about as simply as I can explain it