As reported by Android Police, if you're part of the test then when you look up show times for a movie in Chrome you'll see the usual options to book tickets at different theaters plus a new addition. The "Buy Tickets" button with the Google Assistant logo indicates that Duplex is available. If you click it, you're taken to the movie theater's website but with an Assistant overlay which automatically fills your saved information such as contact information and payment details.

You still need to interact with the website to enter how many tickets you want and to select seats, but most of the other fields are filled out for you. The feature is reportedly working with AMC and Fandango theaters in the US, and will likely make light work of booking other services services such as renting a car in the near future.