If you sign up through a form or landing page, and the double opt-in is enabled, it will send an incentive or confirmation email that needs to be confirmed to be part of your list. If you'd like to auto-confirm your subscribers, you can disable the double opt-in, but we don't recommend it. You can also check this guide for reference:
https://help.convertkit.com/en/articles/2971364-the-all-important-double-opt-in44