Are you talking about the end of the book? If so, here's what happens:
Ralph and Piggy, the only ones left of their group after the other boys have all joined Jack's tribe (either because they wanted to or by force, as with Sam'n'Eric), go to see Jack to try and get Piggy's glasses back.
When Jack sees Ralph and Piggy approaching, he orders the group to prepare to release the boulder on Castle Rock through the operation of a lever - presumably to kill Ralph. It ends up killing Piggy instead by sending him off the cliff. Roger is the one who releases the lever.