Hello. Thank you for contacting the Toolset support.
The custom post field description you add when you add the custom field with your custom field group is intended to display as Backend Description. So, when you add/edit post in backend you will see the description you added for your custom field in the backend post add/edit screen.
The description you added with custom field will not appear in your Toolset Form.
You can add a description in the CRED form itself when you create the form. By adding your custom description text just before your field either using Expert mode or you can add 'HTML content' field.
