A workflow is a work process. That is, a sequence of related steps performed one at a time. For your website, store or landing page, it can be a workflow like this:
Visitor comes to your page
Visitor signs up for your newsletter (by the way: now he is no longer a visitor, but an email subscriber)
Subscriber receives a welcome email
Subscriber will receive an email in the next 3 days with tips and prompts to buy your product
etc.