Suppose we have two animals, X and Y and that X helps Y thanks to a gene. In this equation, we have that B is the benefit that Y receives, r the degree of relatedness and C is the cost of help. If the equation above holds, we have that the benefit (accounted for relatedness) overweighs the cost and the gene will spread. More specifically, the benefit to an individual's fitness (accounting for the probability that he has the gene) is greater than the cost to X's fitness and thus the probability that the gene propagates to the next generation is increased.
