There is a fight between Jack and Ralph, but what happens during the fight?
The fight that you are referring to happens in chapter 11. Ralph, Piggy, Sam, and Eric decide to go up to Jack's camp and confront him about the theft of Piggy's glasses. Ralph wants Jack to return the glasses.
Ralph's attempts to stay non-violent don't work out, and he and Jack start fighting with spears. During the fight Ralph continues to try and convince Jack about the importance of the signal fire. Jack isn't having any of it, and orders his followers to tie up Sam and Eric.
Ralph is enraged by this and begins fighting Jack with renewed fury. While everyone is watching the engagement, Roger pushes a big boulder down from his perch. Ralph is able to dodge it, but Piggy isn't so quick and lucky. He gets hit with a glancing blow and is knocked off of the cliff and dies on impact. Piggy's death is what happens during the fight.
After Piggy's death, Jack and the rest of the tribe turn on Ralph and start throwing spears at him. Ralph barely escapes with his life.