If you wish to allow your shoppers to pay for their orders with PayPal, you will establish that here. To enable this feature, click the Install link as denoted in the image above. You will then be presented with:

Typically, the URL will be https://www.paypal.com/cgi-bin/webscr, Business is your PayPal login ID, and Identity Token is provided by PayPal. Check with PayPal via your PayPal account for the correct values to use here.