The messaging system is what you use to book your accommodation. From this page, you can chat with the renter but also finalize your reservation.
To start with, browse an ad. Then, you can either contact the renter to Ask for more information or Apply. In both cases, you will find a new conversation in your inbox that will allow you to discuss with the landlord. From your conversation with the renter, you can make a Pre-reservation or Accept a booking proposal from the renter to finalize your reservation.
Note: before contacting a renter, make sure your record is complete as he can consult your profile and see whether your record is complete or not.