Freud's The Interpretation of Dreams is a very long book but the essential theory is very simple: dreams are thoughts. While dreaming, we are thinking about stuff, in exactly the same way as we do when awake. The difference is that the original thoughts rarely appear as such, they are transformed into weird images.
Only emotions survived unaltered. A thought about how you're angry at your boss for not giving you a raise might become a dream where you're a cop angrily chasing a bank robber, but not into one where you're a bank robber happily counting his loot. By interpreting the meaning of dreams, the psychoanalyst could work out what the patient really felt or wanted.
The problem of course is that it's easy to make up "interpretations" that follows this rule, whatever the dream. If you did dream that you were happily counting your cash after failing to get a raise, Freud could simply say that your dream was wish-fulfilment - you were dreaming of what you wanted to happen, getting the raise.
But hang on, maybe you didn't want the raise, and you were happy not to get it, because it supported your desire to quit that crappy job and find a better one...
Despite all that, since reading Freud I've found myself paying more attention to my dreams (once you start it's hard to stop) and I've found that his rule does ring true: emotions in dreams are "real", and sometimes they can be important reminders of what you really feel about something.
Most of my dreams have no emotions: I see and hear stuff, but feel very little. But sometimes, maybe one time in ten, they are accompanied by emotions, often very strong ones. These always seem linked to the content of the dream, rather than just being random brain activity: I can't think of a dream in which I was scared of something that I wouldn't normally be scared of, for example.
Generally my dreams have little to do with my real life, but those that do are often the most emotional ones, and it's these that I think provide insights. For example, I've had several dreams in the past six months about running; in every case, they were very happy ones.
Until several months ago I was a keen runner but I've let this slip and got out of shape since. While awake, I've regretted this, a bit, but it wasn't until I reflected on my dreams that I realized how important running was to me and how much I regret giving it up.
While awake, we're always thinking about things on multiple levels: we don't just want X, we think "I want X" (not the same thing), and then we go on to wonder "But should I want X?", "Why do I want X?", "What about Y, would that be better?", etc. Thoughts get piled up on top of one another: it's all very cluttered.
In a dream, most of the layers go silent, and the underlying feeling comes closer to the surface. The principle is the same, in many ways, as this.
But how do I know that feelings in dreams are the "real" ones? In most respects, dreams are less real than waking stuff: we dream about all kinds of crazy stuff. And even if we accept that dreams offer a window into our "underlying" feelings, who's to say that deeper is better or more real?
Well, "buried" feelings matter whenever they're not really buried. If a desire was somehow "repressed" to the point of having no influence at all, it might as well not exist. But my feelings about running were not unconscious as such - I was aware of them before I had these dreams - but I was "repressing" them, not in any mysterious sense, but just in terms of telling myself that it wasn't a big deal, I'd start again soon, I didn't have time, etc.
The problem was that this "repression" was annoying, it was causing long-term frustration etc. In dreams, all of these mild emotions spanning several months were compressed into powerful feelings for the duration of the dream (a few minutes, although the dreams "felt like" they lasted hours).
Overall, I don't think it's possible or useful to interpret dreams as metaphorical representations in a Freudian sense (a train going into a tunnel = sex, or whatever). I suspect that dreams are more or less random activity in the visual and memory areas of the brain. But that doesn't mean they're meaningless: they're activity in your brain, so they can tell you about what you think and feel.