A visitor reaches your website through a Google search or a direct link. At this point the visitor might be reading a post or looking through your products. Sometime during this visit, you offer them the opportunity to sign up for your email list.
If the visitor fills out this form, he becomes a potential customer to whom you can now send offers in multiple ways: email, phone, sms - or all three.
These potential customers return to your site when you contact them with special offers, information, discount codes or other messages carefully crafted to attract them.
The sales funnel narrows as visitors go down it: this happens on the one hand because you will always have more interested people at the top of the funnel than buyers at the bottom of it, but also because your message has to become more and more specific.