An opt-in list is a collection of email addresses you gather from people who agree to receive email messages from you.
“Double opt-in” has become the standard type of signup for agreeing to receive emails.
In the double opt-in process, a person fills out your online signup form with her email address. She then receives an automatic email message asking her to click on a link if she agrees to receive emails from you. Since she must complete these two actions to sign up for emails from you, it's called a double opt-in.
The double opt-in process is designed to protect publishers from being accused of sending spam.
One of the goals for your website is to have repeat visitors.
A great way to get people to come back is to gather the email addresses of your targeted visitors, and remind them of your site and its wonderful content through a series of emails and / or a regular ezine.
You can also announce your newest products (ebooks, etc.) to your opt-in subscriber list, and occasionally promote other people's related products that earn you a commission.