I am not understanding what was highlighted, why requires all players to cooperate? what is Pareto efficiency to do in here?

Note that when analysing social dilemmas it is often convenient to refer to one of the strategies as cooperate and the other as defect. To cooperate is to do what is in the collective interest. So, hauling leaves, remaining silent and working on the group project are examples of a cooperative strategy. To defect is to do what is in a person's individual interest. So, burning leaves, confessing and not working on the group project are examples of defect. Pareto efciency, as we have seen, requires all players to cooperate. But each individual has an incentive to defect