Lewis is the French Dauphin, which means he's supposed to inherit the French throne. As we know, this never happens because Henry V invades France and forces the French to recognize him as the heir to the crown.
We can't say we're sad about this because young Lewis is kind of a brat – he's cocky, condescending, and tends to think that King Henry and his soldiers are a bunch of clowns. This becomes clear when the Dauphin sends Henry an insulting gift, a treasure chest that's full of tennis balls. The message is pretty clear – Lewis suggests that Henry, who has just demanded some minor French dukedoms, is not to be taken seriously. Check out "Symbolism" for more on this.
The problem is that Lewis has seriously underestimated Henry, who is no longer the "vain, giddy, shallow, humorous youth" he once was (2.4.1). The Dauphin reveals his ineptness when he fails to recognize Henry as a threat to France, which suggests that he's not exactly fit to rule.