It is expressing how an analogy can be appropriate and effective in expressing ideas- equally or more so than saying the ideas themselves. 

It pinpoints imprisonment in particular. Perhaps because this is something that can be universally felt and experienced. In relation to the analogy, this means that it was not only those who had the plague that felt imprisonment, it was all those in Oran. And in the reality of occupation all people felt imprisonment as well as those who were prosecuted. 

The second section I don't understand as much, saying it is reasonable to “represent anything that exists by that which exists not” It depends on the definition of exist. It may be referring to all analogies as thing that don't exist therefore saying that the way Camus has written the book is equally as reasonable as people using analogies to further explain or communicate something. 

I think the epigraph could be there as a hint for the analogy that follows but I don't know if this is necessary given that the readers would have just experiences feelings like those in Oran and would understand what the book is truly about. However, the book's analogy may be unclear, and my understanding could be simply because I knew the book's historical context before I read it.

We see imprisonment in the roles that individuals have in society. Whether they be the doctor, priest or housewife, the roles of the village's inhabitants don't change much throughout the novel. This includes the poor and rich, we hear in pg 183 that “poor families consequently found themselves in a very difficult situation, while the rich lacked for practically nothing” It goes on say the “the plague should have worked for greater equality among our fellow citizens…but in fact it heightened the feeling of injustice.” This shows that despite the seemingly universal imprisonment of the plague, it was more devastating for the poor while the rich could manage it better. Though some had to adapt with the arrival of the plague, they went back to their positions afterwards.Though this is continuity it is not always imprisonment- the women characters may seem to be imprisoned by their housewife roles, unable to pursue a career and another way of life, but Dr Rieux's wife has a sense of solidarity in that role. 

We also see freedom in characters like Cottard who benefitted from the plague as it helped him to escape from the imprisonment that his imminent arrest brought. It also might have stopped him from making another suicide attempt.

In today society, we compare things all the time to stuff that don’t actually exist (hasn’t existed or has existed once before). We are in a world where we are consumed by technology, but we want more. We are waiting for the day to come when robots can do our dishes, when they can do our work for us, etc. We are comparing this to what we have now. In the past, we are, as a very specific example, comparing Donald Trump, someone who quite frankly exists, to Adolf Hitler, who once existed. The only thing here is, can we say that Donald Trump is representative of Adolf Hitler, because isn’t this what Daniel Defoe is ultimately saying? That something that already exists can represent something that does not exist. Essentially, Defoe is saying that Camus is representing something that does exist through something that doesn’t exist; the Nazi Occupation of France and ‘The Plague’ in the novel. 

In one sense, ‘imprisonment’ represents the Nazi Occupation of France and ‘The Plague’ in Oran. However, I also think that the choice of the word ‘imprisonment’ can be linked back to how members of the town, everyday people, are trapped with how to think, feel, and react. They have no voice, they are essentially silenced, not just because of the disease, but because they are everyman who do not have power. For example, in the novel, Camus describes the towns people as ‘like nothing. Or, if you prefer, they looked like everyone, part of a general scene.’ They were just people in the background who had no power, no voice, who just carried about their everyday lives because that’s all they could do with no power. In other words, they were trapped. Linking this back to today, we all have a voice, but very fairly are our voices be heard. Do you think we are a silenced community that has no power and are therefore imprisoned in our own society? I think it’s debatable but in some sense we are.