The ! caution sign in front of a guest name in the Bookings overview means that this booking is unconfirmed.
This can be due to several reasons:
the guest has started the booking process and stopped at one point
the payment was just made and is still being verified - please give it up to 24 hours for the payment to be approved and reflected as confirmed in the bookings overview
the used payment method by the guest wasn't approved (e.g. insufficient funds, wrong input of numbers)
To easily follow up on unconfirmed bookings, you can use the option "Send Unconfirmed Reminder" in the dropdown options under Actions on the right side of the guest name.
The guest will receive an automated email notifying them of their incomplete reservation with a unique link to complete payment. It's also mentioned that if the payment won't be completed, the booking will be automatically canceled. The email will also give the option to cancel the tentative booking.