A cyclical structure of a novel means that it ends where it begins. In the beginning of the novel, George and Lennie are migrant farm workers headed for yet another California farm. They are in a sort of idyllic countryside, and Lennie sticks his head underwater like a happy dog. George scolds him and tells him what water to drink, and they talk about the dream George has invented about them owning a rabbit farm. Lennie finds a mouse and accidentally kills it, stroking it because it’s soft.
At the end of the novel, Lennie accidentally kills Curley’s wife by stroking her hair because it’s soft. George runs away with him. In this way, the novel begins as it ends. Lennie accidentally kills something, and George takes him away and shoots him so he won’t accidentally kill anyone else.