Page → Email Collection → Ask new subscribers to confirm their address is on. That person signed up, your site emailed them a link, and they have not followed it and pressed Confirm my address yet. Until they do they are not on your list: they are not counted as a subscriber, they are not passed to Mailchimp, and the launch email does not go to them.

They may not have seen the email yet. It can land in a spam or promotions folder.
If they sign up again on your page more than an hour after the last email, the link is sent again. Within the hour nothing more is sent.
If nobody is receiving the email at all, a red note at the top of the Subscribers screen says so. Check delivery with Send test email; once a test email goes out, the note goes.
Nobody confirms within 30 days, and the address is deleted. The link in the email stops working at the same time; signing up again sends a new one.
You can Remove a waiting address yourself at any time. The Waiting for confirmation tab on the Subscribers screen lists them all on their own, newest first.

People who were on your list before you switched confirmation on are never waiting: they stay on the list as they were.

Why is someone Waiting for confirmation?