What does this equation mean?

The first is cultural group selection, formalized by Robert Boyd, Herbert Gintis, Samuel Bowles and Peter Richerson. Their starting problem is that punishing free-riders is itself a public good — everyone in a group benefits if free-riders are deterred, but only the punisher pays the cost of deterring them, so a population of willing punishers should in principle be invaded by cooperators who never bother to punish, a “second-order” free-rider problem sitting on top of the first [ 9 ] . Their proposed way out rests on an asymmetry between the two roles. A plain cooperator who does not punish pays a fixed cost, call it c , simply by cooperating, regardless of how many other group members defect. A punisher, by contrast, only pays a punishment cost when there is actually a defector present to punish, so as defectors become rare within a group, the punisher’s fitness disadvantage relative to a non-punishing cooperator shrinks toward zero even while the plain cooperator’s disadvantage relative to a defector does not. Writing x for the frequency of cooperators, y for the frequency of punishers, and k for the cost of punishing a single defector, the punisher’s expected fitness penalty is approximately