"Pending tickets" are tickets within orders that are undergoing the checkout process. They might be in the middle of purchasing, or perhaps they've forgotten to finish. Whenever you 'test' your event checkout you would also create a pending ticket.
Pending tickets will disappear automatically when either 1 of 2 things happen
A buyer completes their checkout - OR -
The checkout time expires and the tickets are restocked (default is 20min)
Checkout timeout can be set between 5mins - 59mins via advanced > settings on the left-hand menu bar.
What will pending tickets affect?
Pending tickets still count towards your ticket capacities, since these tickets are "held" by the system to prevent overbooking. This can cause your tickets to:
Appear sold out
Trigger the waitlist
Pending tickets will also include any tickets you have added to the cart during testing of your event after clicking preview