When the Dauphin of France wants to let Henry V know that he's got zero respect for him and his recent demands for some French dukedoms, he sends the English king a giant chest full of tennis balls (1.2). As you've probably guessed, this isn't exactly a friendly invite for Henry to come over to the French palace to practice his backhand stroke. It's a major, major insult that's right up there with "thumb-biting" in Romeo and Juliet.
By sending a gigantic chest of tennis balls, the Dauphin is basically telling everyone that he thinks Henry is immature and would be better off playing a meaningless game than getting involved in messy foreign affairs. The insulting gift also gestures at the fact that Henry spent the "wilder days" of his youth running amok through England instead of paying attention to politics (1.2.10). (Remember all of the ridiculous pranks Prince Hal played back in Henry IV Part 1?)
Henry recognizes the insult immediately and, using the language of tennis, declares that his army is going to destroy all the royal "courts" of France during their deadly "match."
When we have march'd our rackets to these balls,
We will, in France, by God's grace, play a set
Shall strike his father's crown into the hazard.
Tell him he hath made a match with such a wrangler
That all the courts of France will be disturb'd
With chases. (1.2.10)
Yikes! Henry makes tennis and warfare sound pretty scary, don't you think? Note to self: Never give this guy sporting equipment for his birthday.