Rosencrantz and Guildenstern are summoned by King Claudius to distract Hamlet and to find out the cause of Hamlet's seeming madness. King Claudius has employed these two men because they are old childhood friends of Hamlet, and he assumes, therefore, that Hamlet will trust them and confide in them. The theme of appearance versus reality is therefore integral to the scene because Rosencrantz and Guildenstern must appear to be innocently talking with Hamlet, when the reality is that they are there on Claudius's orders to get information from Hamlet.
King Claudius believes that Hamlet might be ambitious enough to take the throne from him and thus become the king of Denmark. Rosencrantz and Guildenstern try to discover whether this is true by talking to Hamlet about ambition. They tell him that, in their opinion, ambition is "so airy and light a quality that it is but a shadow's shadow." It's not possible to know whether Rosencrantz and Guildenstern really believe that ambition is as poor a thing as they here claim, but they must appear to think that it is in order to find out if ambition is the cause of Hamlet's seeming madness. In other words, Rosencrantz and Guildenstern must put on a perhaps disingenuous appearance in order to discover the reality of Hamlet's condition.
When Hamlet asks Rosencrantz and Guildenstern why they have come to Elsinore, they tell him, "To visit you, my lord; no other occasion." Here we know that they are lying. They do have another "occasion" for visiting Hamlet, and that is to find out, on behalf of the king, what Hamlet's intentions are. Hamlet sees through this innocent appearance that Rosencrantz and Guildenstern try to project and asks them directly, "Were you not sent for?" He implores them, as old friends, to "deal justly" with him, meaning that he wants them to answer him honestly. After some hesitation, Rosencrantz and Guildenstern admit that they are there with Hamlet on the orders of King Claudius. The idea here is that appearances are fragile, especially when created for false or disingenuous purposes.