A special offer can be sent to a guest who has sent an enquiry, by using the "special offer" link.
Special rates can be set in various ways, like discounts for longer stays, early bird, last minute or even manually set in the calender.
Or set special price for certain weeks, months or custom time periods. Or by using promotions offered by Airbnb. You should consult the HELP files for all options and how to set.
Hope this helps,