1.) In the beginning of act 2 scene 1, Hamlet, Rosenerantz, and Guildenstern,bare all talking about the future. Hamlet says, "a dream itself is but a shadow". The shadow symbolizes darkness, the darkness also means that Hamlet has no idea what the future has in store for him. Also, since Hamlet has, "gone crazy" it shows how negative he is being lately, and we know something is wrong with him.
2.) Rosencrantz says dreams are, "airy and light". The airy and light could mean that he sees his future clearer and uses it in a more positive assumption and future plans. This shows us just how different Hamlets and Rosencrantz's perspectives are about things right now.
3.) when Hamlet calls Polonius a "fishmonger", this means he is downgrading him. Polonius has a high reputation, and to call him a fishmonger would be considered a very rude insult. He is implying that Polonius could be dishonest or sneaky, because those are qualities a fishmonger is known for.