At one point in Lord of the Flies, Ralph fears capture by "the Reds." Who are the Reds?
In Chapter 10, there is a deep division between Jack and Ralph, and tensions are building to a climax. They have already taken part, either directly or indirectly, in Simon's murder, and Piggy and Ralph especially are feeling unsettled by the apparent declining morals of the entire group. As they talk, Ralph notes, "We might get taken prisoner by the Reds." The use of a proper noun here directs us to history.
After the Bolshevik Revolution of 1917, the Soviet Communist government created the Red Army. Therefore, Ralph is noting the tensions that exist in his world where Britain has entered a war against Communist Russia. Ralph is afraid that if they are found and rescued, it will be by the enemy of their own country.
Eric notes that even this would "be better than—" but never finishes his thought. The implication is that being a prisoner of the Communists would be better than continuing to live with Jack or possibly with the dead man in the parachute who haunts them.
By "the Reds," Ralph means the Communists, or the Soviet Block.
Golding wrote "Lord of the Flies" in 1954, nine years after the end of World War II, in which he served. It was the time of the cold war. The threat of nuclear conflict between the Soviets and the West was very real (at least in people's minds), and Golding implies that World War III, were it to happen, would result from the hostility between the forces of democracy and communism.
In subsequent comments, Golding has suggested that the novel may be considered a fable about the weakness of democratic societies (Ralph's lot) in the face of powerful dictatorships, such as those of Hitler and Stalin (or Jack). The allusion to "reds" brings us back to that level of interpretation, giving us another, political, level of understanding of the book.
Actually, Ralph is referring to whomever the British are fighting during this time. Remember that the novel is set in the future during an atomic war. We, as an audience, are not given an actual enemy for England, but it's severe enough to warrent evacuation. In chapter 10, Ralph, Piggy, and Samneric are discusses the things they would like to have on the island now. The suggestion of a boat comes up...:
"Ralph dredged in his fading knowledge of the world.
'We might get taken prisoner by the Reds.'
Eric pushed back his hair.
'They'd be better than-'"
The boys are actually comparing what Jack and his tribe have done and are doing to what some foreign country has done to England during this future war. They are deciding who it would be worse to be captured by.