Why am I seeing a "pending payment" appointment in my calendar?

A "pending payment" appointment box in your calendar means someone is in the process of booking an appointment that has a required up-front payment

The appointment will say "pending payment" while your client or customer is completing the payment. 

If they abandon payment, or payment is not completed, the "pending payment" box will disappear, and free up the appointment time slot. 

What you see in your calendar: