Prisoner's dilemma is perhaps the most well known game from the game theory.
The basic premise is that if you and your partner cannot communicate with each other, and you must decide whether or not to betray your partner, while your partner makes the same decision.
When you only look at it from your perspective, the rational thing to do is to betray your partner.
If you betray them and if your partner doesn't, then you get better rewards than had you not betrayed them. Same applies if your partner betrays you.
But when you consider your partner as well, it becomes clear that the best outcome is when both of you work together, and don't betray one another. And this is why this seemingly simple problem is, in reality, a complex dilemma. And we can observe variants of prisoner's dilemma-like scenario in nearly every aspect of the world, and with many variants, this thought experiment has helped us better understand how the world works.