A strategy is strictly dominated when another option yields a higher payoff against every choice the opponent might make, so a rational player never plays it. Once you delete such a strategy, options that were not dominated before can become dominated, letting you repeat the deletion and sometimes narrow the game to a single predicted outcome.
A set of strategies is a Nash equilibrium when each player's choice is a best response to what the others are doing, meaning no single player can gain by unilaterally changing their move. It is a stable point not because it is collectively optimal but because no individual has a private incentive to deviate.
In the prisoner's dilemma, defecting strictly dominates cooperating for each player, so both defect even though mutual cooperation would leave both better off. The pursuit of individually optimal choices can lock participants into a jointly inferior result that neither can escape alone.