Events from your Calendar will be displayed as options for the drop-down list of the form that has been set. This step helps you set up the format so that events can be distinguished from each other so that your customers can easily identify them to choose from. You can study more formats in many languages here
For this add-on to work, your form must include a customer's email field. And this step is for you to enter where that email field is. The add-on will add users to the event of their choice and send them invitations to emails retrieved from this setup step.