This bit comes just after Jack has shouted "Bollocks to the rules!" and run away from the assembly. Ralph and Piggy are left standing there, wondering what to do next. Piggy encourages Ralph to stamp his authority on the assembly and re-call the meeting with the conch.
“Blow the conch, Ralph.”
Piggy was so close that Ralph could see the glint of his one glass.
“There’s the fire. Can’t they see?”
“You got to be tough now. Make ’em do what you want.”
Ralph, however, knows that not everyone respects the democratic principle of the conch in the way that he and Piggy do:
Ralph answered in the cautious voice of one who rehearses a theorem.
“If I blow the conch and they don’t come back; then we’ve had it. We shan’t keep the fire going. We’ll be like animals. We’ll never be rescued.”
“If you don’t blow, we’ll soon be animals anyway. I can’t see what they’re doing but I can hear.”
Ralph is worried that the crack which has been opening up in him and Jack's relationship might become a permanent schism. He knows that he's potentially on the brink of disaster if he doesn't react in the right way.
If he blows the conch, and no-one returns, the conch has lost all its power. "We've had it", he says. If he leaves it, and lets them go, then potentially the conch might continue to work in the future. There might be a chance of reconciliation.
Because, he is worried that the group as a whole, will start to rebel, and there will be no order. Just mayhem.