The Chorus steps on stage and kicks things off with a bang by asking for a "muse of fire" to help the theater company portray "a kingdom for a stage." Translation: Shakespeare means serious business in this play. (This classic move, by the way, is called an Invocation to the Muse. Check out the openings of The Odyssey,Paradise Lost, and The Aeneidfor some other famous examples.)
Then the Chorus tells us to get ready to rumble because we're about to watch "two mighty monarchies" (England and France) go toe-to-toe over the French crown.
We interrupt this program for an important brain snack: When we say "the Chorus," we're not talking about the kids from Glee. In Henry V, the Chorus is a single character who sets the scene for us at the beginning of each act. It's a throwback to the old school Choruses (a group of singers that act like a peanut gallery) that we see in Greek tragedies like Antigone and Oedipus the King. Now, back to our show.
After pumping us up for some serious drama, the Chorus then apologizes because there's no way the theater can accurately represent Henry V's war against France on a tiny little stage with just a handful of actors.
The Chorus tells us that we have to use our imaginations to help bring the events to life, since it's impossible for the theater to "hold the vastly fields of France" and the thousands of soldiers and horses that were involved in the historic battle at Agincourt.