I having a problem in setting up the paypal link. What is a client secret?

The client secret is asked when you're integrating with PayPal checkout, this guide will provide you more details about it: https://www.jotform.com/help/542-How-to-integrate-PayPal-Checkout-with-your-form 

If you want to integrate with PayPal standard, then you may refer to this guide instead: https://www.jotform.com/help/391-How-to-Integrate-Form-with-Paypal-Standard-