The antagonism between Jack and Ralph, who can be regarded as the novel's main characters, forms - in my view - the real thread of the narrative.

thread 的本义是"线、细线"，在这里用作比喻意义，指"贯穿故事的主线或线索"。就像一根线把珠子串起来一样，thread 在这里比喻把整个故事串联起来的核心要素。