What does Piggy mean when he says, “I know about people. I know about me. And him. He can’t hurt you: but if you stand out of the way he’d hurt the next thing. And that’s me.”

Piggy is rationalizing his standing on the island. Piggy is keenly aware that Jack is a consummate bully. He goes after the weak and that would be piggy. Piggy knows that Ralph's actions have consequences to him. Jack would not expend the energy nor have the courage to challenge Ralph. Instead, when frustrated, he simply picks on the weakest option.