The pre-rental email is sent to your customers at 6 AM on the morning a reservation is scheduled to take place. This serves as a useful reminder, particularly if the reservation was made far in advance. 

Enable the pre-rental email by going to Settings > Notifications > What and turning on the "SendPre Rental Email" slider.

Set a subject and content and you're done! The reservation details will be included automatically in the email sent to the customer, below the message you enter in the email content text box.