In Lord of the Flies what does Simon mean when he says what I mean is maybe it's only us?

Simon's comment was about the beast. He suspected that the beast was not an animal and obviously didn't believe that it was a ghost. This left only one remaining possibility, that there was no beast. Simon believed that the beast was simply a product of the boy's own imaginations. The beast only existed in their own minds. However there was a deeper reasoning to Simon's comment. Simone suspected that the beast wasn't just a product of the boys' own imaginations but was actually the boys themselves. It was the boys themselves who were the 'beast,' capable of hurting, of killing and of committing acts of evil.