Dream imagery appears often in A Clash of Kings; in fact, Bran's story is approximately eighty-seven percent him dreaming. Thankfully, his dreams are a little more interesting than ours, or a big chunk of this book would be characters going to school and realizing they're only wearing their underwear.
Deconstructing dreams in A Clash of Kings is a difficult prospect, though, since there are different types of dreams. On the one hand, we have the vanilla type of dream—that is, dreams that represent a character's deepest desires. When Catelyn dreams "that Bran was whole again, that Arya and Sansa held hands, that Rickon was still a babe [and that] Robb, crownless played with a wooden sword" (23.Catelyn.1), we get an insider's view of Catelyn's ambition, at what motivates her to act as she does and despair when this scenario becomes less and less likely.
On the other hand, we have types of dreams that are more difficult to analyze. First there are the dreams that have a physical affect on the world, such as Bran's wolf dreams. When Bran sleeps, he enters the body and mind of his direwolf, Summer. During these midnight excursions, Bran thinks "[p]art of him knew it was only a dream, but even the dream of walking was better than the truth of his bedchamber, walls and ceiling and door" (5.Bran.81). These dreams reveal desire, because Bran wants more than anything to walk again.
But instead of staying in the realm of desire, unlike Catelyn's dream above, Bran actually walks in these dreams, and his actions actually have consequences in the world. If Bran kills a man in his wolf dream, that guy is dead. If Bran doesn't wake from his wolf dream for days, his body suffers the consequences of not eating or moving.
Next up there are dreams that foretell the future. Jojen is the main player here with his green dreams. Green dreams prophesiy the future, but in typical oracle fashion, they only do so with enigmatic imagery. As Jojen exclaims, "The green dreams take strange shapes sometimes. […] The truth of them is not always easy to understand" (36.Bran.28). Dany has similar dreams in the House of the Undying, seeing images such as a "feast of corpses" and a "blue flower [that] grew from a chink in a wall of ice" (49.Daenerys.34, 76). Again we're left to decipher.
And to be honest, it's hard to peg down an interpretation for these types of dreams. Perhaps they represent the effect dreams have on reality—in other words, before something can change in the real world, someone has to dream it first.
But if this is the case, then do Jojen's dreams change the world, or do changes in the world alter his dreams? Bran's wish to walk seems to be the catalyst that allows him to skin change with Summer, but Jon might have the same latent abilities. So are their dreams symbolic? Or are they merely plot devices?
Ultimately, by the end of A Clash of Kings, the symbolic and narrative importance of dreams hasn't been fully fleshed out, so we'll have to read the other books to see how important they prove. But the imagery is definitely present, so keep a look out of dreams as you further explore A Song of Ice and Fire.