Clearly in this book there are a number of signs and omens that are presented that indicate something of the future, in particular relating to the conflict between Odysseus and the suitors and his eventual triumph. Perhaps one of the main examples we can identify is when at dawn Odysseus asks Zeus for some sign of what is to happen in the ensuing conflict. The peal of thunder that is his answer in the cloudless sky comforts him, as does his overhearing of a young maiden who hopes that this sign will usher in the swift destruction of the suitors.
However, the real function of all of these dreams and signs is that it greatly raises suspense and tension as we await for the final showdown between Odysseus and the suitors that we have all been waiting for for so long. Having various dreams and portents with their customary ambiguous language really serves to make us wonder what precisely will happen and who will triumph.
In Book 20 of the Odyssey, the omens, dreams, and oozing of blood from the suitor's mouth portend the coming clash between Odysseus and the suitors, when he will kill them all for disrespecting his wife and son and the honor of his house by killing all of his beasts, eating all of his food, ruining his home, and trying to marry his wife. Odysseus returns home and stays in disguise to see what has been going on in his absence. He has his patron Athena in his corner, and his son Telemachus is ready to kill all the suitors himself, if need be. Check out the study guides and summary on the link below.