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The authenticity token is designed so that you know your form is being submitted from your website. It is generated from the machine on which it runs with a unique identifier that only your machine can know, thus helping prevent cross-site request forgery attacks.
If you are simply having difficulty with rails denying your AJAX script access, you can use
<%= form_authenticity_token %>
to generate the correct token when you are creating your form.
You can read more about it in the documentation.