Ralph and Jack are at odds over their mutual desire to lead the boys on the island. Ralph becomes the default leader through his possession of the conch. Jack, as the leader of the choir, thinks he should be in charge of everyone. Ralph tries to diffuse the situation by making Jack's choir the hunters, which works for awhile. Jack is happy being in power and providing the rest with food. However, this blows up when Ralph, Simon, and Piggy are the only ones building shelters. They almost physically fight, but both manage to pull it back together.
It essentially becomes a fight over responsibilities, & it threatens the very shaky truce established after Ralph was voted leader. It is the first time Jack and Ralph have voiced their dissatisfaction with one another, and it sets the tone for the rest of the novel. So this is the first wedge driven into their friendship, & their mutual respect quickly becomes mutual distrust and hatred. The true source of the conflict is Jack's inability to let anyone else have power. Ralph truly wants what's best in terms of survival, although he doesn't always behave according to those thoughts. Jack loves power, and only seeks to have more and more.