Discuss the Prisoner's Dilemma.
A more specific question would be likely to get you a better question as we cannot be sure of what aspect of the prisoners' dilemma you would like us to address.
The most important thing to understand about this game is that it yields different results if it is done once or a few times as opposed to if it is done many times. It is vital to understand this because the difference has a major impact on how useful the game is for understanding real world events.
In International Relations, for example, we often talk about this game as an indicator of how countries act towards their rivals. It is used to help explain why countries might end up going to war. But we must realize that a game that is played many times (if countries interact with one another many times) is very different than a one-off game. The two sides can come to know one another and maybe even trust one another to some degree. They also cannot simply play for the present situation but must plan for the future because they know they will be "playing" again at some point. This makes their calculations very different than they would be if the game were a one-off.