Ever watch a WWE wrestling match, where an announcer comes out on stage in a fancy suit or a tuxedo and gets the crowd warmed up by saying stuff like, "Let's get ready to Ruuuuumbleeeee!"? If we think about it, getting the play-going audience pumped up and ready for some serious action (read: England's big throwdown with France) is a big part of the Chorus's job in this play. Check out this passage from the first Prologue:
And let us, ciphers to this great account,
On your imaginary forces work.
Suppose within the girdle of these walls
Are now confined two mighty monarchies, (Prologue.18-21)
Once the play gets going, Shakespeare's Chorus turns into something like a Super Bowl color announcer, which is pretty awesome if you think about it. At the beginning of each act, the Chorus sets the scene, comments on the action of the play, and helps to identify important themes. It's basically a throwback to the kinds of Choruses (a group of singers that acted like a peanut gallery) we find in ancient Greek drama like Antigone and Oedipus the King.
Here's the catch. In Henry V, the Chorus is really self-conscious. At the beginning of every act, the Chorus apologizes because the theater can't do justice to the historical events it's trying to represent on stage. So, on the one hand, the Chorus tries super-hard to get us excited about watching (or reading) Henry V. On the other hand, the Chorus keeps telling us that the theater stinks. What's up with that? Let's check out these lines from the Prologue, where we're told that that we're about to watch "two mighty monarchies" go toe to toe at the Battle of Agincourt:
Can this cockpit hold
the vastly fields of France? Or may we cram
Within this wooden O the very casques
That did affright the air at Agincourt? (Prologue.12-15)
The answer, in a word, is no. It's impossible for Shakespeare's company to "cram" thousands of actors/soldiers into one tiny "cock-pit" (theater). That's why we're asked repeatedly to use our imaginations to help compensate for what can't be portrayed: "Think, when we talk of horses, that you see them, / Printing their proud hoofs i' the' receiving earth" (Prologue.27-28). To a large extent, the success of the play will depend on the audience's willingness to participate. It helps that the Chorus uses such vivid language because we can totally imagine thousands of horses hooves pounding into the earth as soldiers ride across the "vastly fields of France, even if we're just looking at a small stage with a handful of individual actors who each represent "a thousand" soldiers.