Pending can mean a few things. If it’s a front-end booking, it probably means the customer chose a manual payment type – or they selected PayPal payment but then didn’t complete their transaction. Either way, you will need to contact the customer about completing their payment or take payment on the day of the trip. It can also mean the option itself was set to require confirmation – normally a front-end booking changes from Pending to Received when payment is completed, but it is possible to override that at the option level.
More likely it’s a back-office booking, in which case it means that the agent who entered it didn’t change the status from Pending to Received. That might be because payment has yet to be completed or it might be because they just forgot to change the status. You can see from the transaction list on the booking details page if there is still money owed on the booking.