There is a lot that we have to piece together from the rather startling account that Chief Bromden gives us of the asylum where he is based. So much of what he sees and accepts as reality is not actually reality, but rather symbolises his understanding of what is going on. However, before we dismiss what he says, as we read the novel more closely we see that everything the Chief reports does have a correspondence to reality, even if it does not have the same form as Chief Bromden gives it in his account.
He imagines the asylum where he lives to be some kind of highly controlled environment ran by the Combine, which is a name that Bromden gives to symbolise the oppressive forces of authority and society that have placed him, and all his fellow inmates, inside the asylum. Note the way that Bromden talks about patients who are taken away for "treatment" and are returned as a "success story":
A success, they say, but I say he's just another robot for the Combine and might be better off as a failure, like Ruckly sitting there fumbling and drooling over his picture.
The Combine, then, seems to represent the forces of society that want patients such as Bromden controlled and kept under lock and key. Of course, the chief representative of the Combine in the novel is Nurse Ratched, whom Bromden imagines installing machinery inside of patients to keep them under control. Although this is clearly false, Nurse Ratched is shown to maintain her control through a terrifying mix of cruelty, manipulation and suggestion that operates just as efficiently as the imagined machinery Bromden perceives. Bromden does his best to try and avoid the attentions of the Combine, and his mist that he dreams up is a defence mechanism to protect him from the attentions of the Combine. He is terrified by the idea of losing control to the Combine like others have before him.