The Payment Form is a page on which clients can make a payment.
Each booking has its own payment form URL, which can be viewed by clicking on View Payment Form, to find when you scroll down the booking page in the outer right corner:
An example of the Payment Form from the booking showed above:
Please note that you can send the Payment Form to a client by using the Payment URL tag in one of your templates (Settings > Automation > Email Templates).
The Payment URL can be included in any kind of Template.
If you would like to know how to request a specific amount using the Payment URL please check this article.