I assume that you are talking about the part in Chapter 14 where Huck reads the books the have about royalty and then he and Jim discuss the subject a bit.
Their conversation makes it clear that Huck does not really know very much about kings and dukes and royalty. He talks, for example, about kings executing everyone in their parliaments and spending most of their time in their harems.
I think that this is meant in part to show how Huck is a real American with the aggressively republican attitudes of his time. Americans do not know about royalty because we never had such things and Huck cannot really imagine what royalty would be like.