Hi there,
That email is the incentive email. If you have the double opt-in enabled on your landing page or form, it will send out an email to your subscribers when they sign up to confirm their subscription and be part of your list. If you don't want them to receive the email and be automatically confirmed, you can disable the double opt-in of the form but we don't usually recommend it.
You can check this guide for reference:
https://help.convertkit.com/en/articles/2502655-the-incentive-email37