Well -- that comment was not targeted at a layperson, I think. But the idea is: the scattering matrix (https://en.wikipedia.org/wiki/S-matrix https://en.wikipe

Well -- that comment was not targeted at a layperson, I think. But the idea is: the scattering matrix (https://en.wikipedia.org/wiki/S-matrix https://en.wikipedia.org/wiki/S-matrix) is a matrix that says for each possible input and output state, what the amplitude of that transition happening. The perturbation expansion is roughly a Taylor series in the number of interactions that occur in a scattering problem. So: no interaction is order-0, exchanging one particle is order-1, etc. The first dozen terms or so then tell you the dozen or so highest-amplitude processes that occur in a given interaction. Most of these will involve virtual particles.