The Prisoner's Dilemma is an old puzzle with societal implications. Imagine two prisoners, Achilles and Manfred against whom there is little real evidence. Although these prisoners have committed a crime, they can expect little punishment (say one year in prison) if they both remain silent. If Achilles confesses to their crime, however, Manfred remains silent, Achilles will be released while Manfred can expect a five year sentence. Conversely, if Manfred talks and Achilles remains silent, Manfred will be released while Achilles can expect a five year sentence. Thus each man has a choice of confessing or not. If they both confess, each man gets a three year term; if they're both silent, each gets a one year term; if A confesses and B is silent, A gets off and B gets five years.
The optimum course of action is to stay silent. in general, that is not the course that Achilles and Manfred are likely to follow: they are both likely to confess so as to avoid being a patsy for the other. This situation is not limited to prisoners. Spouses in a marriage, businessmen in a competitive market, and national governments in an arms race can all fall subject to such prisoner's dilemmas.