-Lines 1-20 of act 3 scene, 1.
This is the conversation between Claudius, Gertrude, Rosenercrantz, and Guildenstein. It is interesting because they are all very worried about Hamlet. They are tying to decipher his bizarre behavior that they are not used to. This meeting of these characters is what's starts out this while act, so you know it has a large significance on the future events.
This long speech is significant because we know this is the test of Polonius and Claudius to see if he loves Ophelia. hamlet reveals his true thoughts and feelings about her and things, so this is a important part in this story.