These are known as "Wait-listed bookings".
They appear in ResBook as a yellow stripe at the top of an existing booking, like this:
Hover your mouse cursor over the booking on the Calendar to see what is wait-listed. Click on the Reservation ID number to open the wait-listed booking.
By default, ResBook does not prevent guests from placing booking requests on days which are already pending, but it will prevent them from requesting on top of instantly confirmed bookings. You retain the ability to accept or decline booking requests.
Before placing a booking request the guest will have seen your availability on your calendar, so they should be aware of your occupancy.
Even if you are unable to accommodate additional guests, it is an opportunity to open a dialogue with prospective guests and recommend more suitable alternative dates, or other available rooms according to availability.
The underlying intent is that ResBook should not prevent potential guests from contacting you. This has proved helpful in cases where guests have inadvertently selected an incorrect arrival date!