Do I need to manually accept pending submissions in order to process payment in Paypal?

Most of our clients are having trouble submitting payment in PayPal. They have to try several different times and change the browser in order to make it.

It has been quite a trouble. Now I realized there are 63 pending submissions.
What does "complete submission" mean? I need to do it in order to receive the payment?

Each submission on a Paypal-integrated form is initially considered an Incomplete Payment entry. JotForm uses the IPN message to determine whether a submission's payment is successful or not. If successful, the submission is marked as completed -- the post-submission actions are then performed -- such as sending out the notifier and autoresponder emails and sending the submission data to WebHook URLs (if there is any). 

If the payment is failed in any case, the submission remains in incomplete payments. Clicking on Complete submission button will not process the payment, but will trigger the post submission actions such as sending notifications/auto-responders.

Please refer to the guide below that can help you with more information on Incomplete payments.

https://www.jotform.com/help/272-Paypal-Incomplete-Payments-FAQ

The customers will need to submit the form again and complete the payment in order to receive the payment again.

Hope this information helps!