What is the Prisoner's Dilemma and how does it relate to Cassia and Ky?

The next day, on the hike, Ky talks with her about the Prisoner's Dilemma. The only way to stay safe is if neither prisoner betrays the other one. They get as close to each other as they can, but they know they cannot kiss each other except for on the cheeks. Once they truly kiss, they have committed an Infraction.