The Proxy URL lets the OAuth providers (Google, Yahoo or Outlook.com) know that you are asking CloudSponge to get the address book for you. The sole purpose of this page is to accept the OAuth code parameter from the provider and “proxy” it to CloudSponge.
The OAuth flows provided by Google, Yahoo and Outlook.com are designed for a site like yours to request access to a person’s address book. After the person grants consent, they are redirected back to your app. In order for CloudSponge to be able to do the heavy lifting of importing and normalizing the contacts for your site, your application must hand the consent code over to CloudSponge.