The idea is that if suckers help everyone, cheats can do better, accepting help but not giving it in return. In this case, they would do better in survival and reproduction and increase in number. Soon there would be only cheaters. But at a certain point, the disadvantages of cheating outweigh the advantages, so the proportion shrinks. The optimal frequency is proportion is the one on which the process stabilises.
Proportion is relative frequency.
So this point depends on frequency.