In order for a web site powered by Recite CMS to communicate with the Recite CMS installation a backend request driver is used. Common situations where this would arise are as follows:
When a user submits a form, the forms backend request handler processes the form.
When an authenticated user wants to log out the users backend request handler accepts this request and logs out the user.
When a user adds a product to their shopping cart the e-commerce backend request handler deals with this.
If you want to enable any kind of Ajax interaction between your web site and a custom module, you would use JavaScript to connect a backend request handler.
This guide shows you how to create your own backend request handler.