You may have noticed that the Nash equilibrium
in this example seems suboptimal in a way, since it is possible for both players to get -1 rather than -6. This is a natural outcome of the interaction present in the game- in theory, not confessing would be an optimal strategy for the group collectively, but individual incentives prevent this outcome from being achieved. For example, if player 1 thought that player 2 would remain silent, he would have an incentive to rat him out rather than to stay silent, and vice versa.
For this reason, a Nash equilibrium can also be thought of as an outcome where no player has an incentive to unilaterally (i.e. by himself) deviate from the strategy that led to that outcome. In the example above, once the players choose to confess, neither player can do better by changing his mind by himself.