Each player (prisoner) can either defect (confess) or cooperate (don't confess) to try to maximize his own payoff (low prison term).
For the payoff matrix to qualify as a prisoner's dilemma game, it must be true that
T = "Temptation to cheat" payoff
R = "Reward for mutual cooperation" payoff
P = "Punishment for mutual defection" payoff
S = "Sucker's" payoff